Add missing page-level applies_to for ingest docs (#3238)

## Summary

This PR adds/updates page-level `applies_to` information for:

- [x] docs in the `manage-data/ingest` folder
- [x] `reference/apm-agents/index.md` (+ adds some links)
- [x] docs in the `reference/fleet` folder:
  - [x] "Fleet and Elastic Agent overview" page (`index.md`)
  - [x] "Beats and Elastic Agent capabilities" page
  - [x] "Migrate from Beats to Elastic Agent" section
  - [x] "Deployment models" section
  - [x] "Install Elastic Agents" section
  - [x] "Secure connections" section
  - [x] "Manage Elastic Agents in Fleet" section
  - [x] "Configure standalone Elastic Agents" section
  - [x] "Manage integrations" section
  - [x] "Command reference” page
  - [x] “Agent providers” section
  - [x] "Agent processors" section
- [x] `reference/ingestion-tools/index.md` (+ adds some links)

## Generative AI disclosure

1. Did you use a generative AI (GenAI) tool to assist in creating this
contribution?
- [x] Yes  
- [ ] No

2. Tool(s) and model(s) used:

Cursor / gpt-5.1 model

## Review assignments

| Commit Hash | Commit Title | Reviewer | Done |
|------------|--------------|----------|------|
|
[33ab065](33ab065)
| Add applies_to to reference/apm-agents/index.md | @theletterf | ✅ |
|
[875e40c](875e40c)
| Add applies_to to reference/ingest-tools/index.md | @bmorelli25 | ✅ |
|
[93c0d87](93c0d87)
| Add applies_to to manage-data/ingest docs | @karenzone | ✅ |
|
[ff9a870](ff9a870)
| Add applies_to to agent processor docs | @alexandra5000 | ✅ |
|
[1e4576b](1e4576b)
| Add applies_to to more pages | @bmorelli25 | ✅ |
|
[2c6519f](2c6519f)
| Add applies_to to 'Manage integrations' docs | @bmorelli25 | ✅ |
|
[70857ac](70857ac)
| Add applies_to to 'Deployment models' docs | @bmorelli25 | ✅ |
|
[a008244](a008244)
| Add applies_to to 'Secure connections' docs | @bmorelli25 | ✅ |
|
[1178589](1178589)
| Add applies_to to fleet/index.md | @bmorelli25 | ✅ |
|
[5adeb75](5adeb75)
| Add applies_to to 'Install Elastic Agents' docs | @karenzone | ✅ |
|
[b6e14c6](b6e14c6)
| Add applies_to to 'Manage Elastic Agents in Fleet' docs | @karenzone |
✅ |
|
[7483e33](7483e33)
| Update applies_to of 'Agent providers' docs | @karenzone | ✅ |
|
[a0d4edb](a0d4edb)
| Add applies_to to 'Configure standalone Elastic Agents' docs |
@karenzone | ✅ |

---------

Co-authored-by: Brandon Morelli <brandon.morelli@elastic.co>

Author: vishaangelova

manage-data/ingest/ingest-reference-architectures.md

@@ -2,6 +2,9 @@
 mapped_pages:
   - https://www.elastic.co/guide/en/ingest/current/index.html
   - https://www.elastic.co/guide/en/ingest/current/use-case-arch.html
+applies_to:
+  stack: ga
+  serverless: ga
 products:
   - id: elastic-agent
 ---
@@ -16,6 +19,10 @@ To ingest data into {{es}}, use the *simplest option that meets your needs* and
 You can host {{es}} on your own hardware or send your data to {{es}} on {{ecloud}}. For most users, {{agent}} writing directly to {{es}} on {{ecloud}} provides the easiest and fastest time to value. {{ech}} is available on {{aws}}, GCP, and Azure, and you can [try it for free](https://cloud.elastic.co/registration).
 ::::
 
+::::{admonition} Elastic Cloud Serverless architectures
+Architecture diagrams show {{es}} and {{kib}} as separate components. {{es}} and {{kib}} are combined in your {{serverless-short}} project.
+::::
+
 **Decision tree**
 
 [Data ingestion](../ingest.md)
@@ -27,5 +34,5 @@ You can host {{es}} on your own hardware or send your data to {{es}} on {{ecloud
 | [*{{agent}} to proxy to Elasticsearch*](./ingest-reference-architectures/agent-proxy.md)<br><br>![Image showing connections between {{agent}} and {{es}} using a proxy](/manage-data/images/ingest-ea-proxy-es.png "") | Agents have [network restrictions](./ingest-reference-architectures/agent-proxy.md) that prevent connecting outside of the {{agent}} network. [{{ls}} as proxy](./ingest-reference-architectures/ls-networkbridge.md) is one option.<br> |
 | [*{{agent}} to {{es}} with Kafka as middleware message queue*](./ingest-reference-architectures/agent-kafka-es.md)<br><br>![Image showing {{agent}} collecting data and using Kafka as a message queue enroute to {{es}}](/manage-data/images/ingest-ea-kafka.png "") | Kafka is your [middleware message queue](./ingest-reference-architectures/agent-kafka-es.md):<br><br>* [Kafka ES sink connector](./ingest-reference-architectures/agent-kafka-essink.md) to write from Kafka to {{es}}<br>* [{{ls}} to read from Kafka and route to {{es}}](./ingest-reference-architectures/agent-kafka-ls.md)<br> |
 | [*{{ls}} to Elasticsearch*](./ingest-reference-architectures/ls-for-input.md)<br><br>![Image showing {{ls}} collecting data and sending to {{es}}](/manage-data/images/ingest-ls-es.png "") | You need to collect data from a source that {{agent}} can’t read (such as databases, AWS Kinesis). Check out the [{{ls}} input plugins](logstash-docs-md://lsr/input-plugins.md).<br> |
-| [*Elastic air-gapped architectures*](./ingest-reference-architectures/airgapped-env.md)<br><br>![Image showing {{stack}} in an air-gapped environment](/manage-data/images/ingest-ea-airgapped.png "") | You want to deploy {{agent}} and {{stack}} in an air-gapped environment (no access to outside networks)<br> |
+| [*Elastic air-gapped architectures*](./ingest-reference-architectures/airgapped-env.md) {applies_to}`serverless: unavailable`<br><br>![Image showing {{stack}} in an air-gapped environment](/manage-data/images/ingest-ea-airgapped.png "") | You want to deploy {{agent}} and {{stack}} in an air-gapped environment (no access to outside networks)<br> |
 

manage-data/ingest/ingest-reference-architectures/agent-apis.md

@@ -1,6 +1,9 @@
 ---
 mapped_pages:
   - https://www.elastic.co/guide/en/ingest/current/agent-apis.html
+applies_to:
+  stack: ga
+  serverless: ga
 products:
   - id: elastic-agent
 ---

manage-data/ingest/ingest-reference-architectures/agent-es-airgapped.md

@@ -1,6 +1,9 @@
 ---
 mapped_pages:
   - https://www.elastic.co/guide/en/ingest/current/agent-es-airgapped.html
+applies_to:
+  deployment:
+    self: ga
 products:
   - id: elastic-agent
 ---

manage-data/ingest/ingest-reference-architectures/agent-installed.md

@@ -1,6 +1,9 @@
 ---
 mapped_pages:
   - https://www.elastic.co/guide/en/ingest/current/agent-installed.html
+applies_to:
+  stack: ga
+  serverless: ga
 products:
   - id: elastic-agent
 ---

manage-data/ingest/ingest-reference-architectures/agent-kafka-es.md

@@ -1,6 +1,9 @@
 ---
 mapped_pages:
   - https://www.elastic.co/guide/en/ingest/current/agent-kafka-es.html
+applies_to:
+  stack: ga
+  serverless: ga
 products:
   - id: elastic-agent
 ---

manage-data/ingest/ingest-reference-architectures/agent-kafka-essink.md

@@ -1,6 +1,9 @@
 ---
 mapped_pages:
   - https://www.elastic.co/guide/en/ingest/current/agent-kafka-essink.html
+applies_to:
+  stack: ga
+  serverless: ga
 products:
   - id: elastic-agent
   - id: logstash

manage-data/ingest/ingest-reference-architectures/agent-kafka-ls.md

@@ -1,6 +1,9 @@
 ---
 mapped_pages:
   - https://www.elastic.co/guide/en/ingest/current/agent-kafka-ls.html
+applies_to:
+  stack: ga
+  serverless: ga
 products:
   - id: elastic-agent
   - id: logstash

manage-data/ingest/ingest-reference-architectures/agent-ls-airgapped.md

@@ -1,6 +1,9 @@
 ---
 mapped_pages:
   - https://www.elastic.co/guide/en/ingest/current/agent-ls-airgapped.html
+applies_to:
+  deployment:
+    self: ga
 products:
   - id: elastic-agent
   - id: logstash

manage-data/ingest/ingest-reference-architectures/agent-ls.md

@@ -1,6 +1,9 @@
 ---
 mapped_pages:
   - https://www.elastic.co/guide/en/ingest/current/agent-ls.html
+applies_to:
+  stack: ga
+  serverless: ga
 products:
   - id: elastic-agent
   - id: logstash

manage-data/ingest/ingest-reference-architectures/agent-proxy.md

@@ -1,6 +1,9 @@
 ---
 mapped_pages:
   - https://www.elastic.co/guide/en/ingest/current/agent-proxy.html
+applies_to:
+  stack: ga
+  serverless: ga
 products:
   - id: elastic-agent
 ---