docs: add missing documentation for decimal datatype and protocol version 3

[RaphDal]

This pull request adds documentation for decimal datatype support in QuestDB, specifically for protocol version 3. The changes clarify how to ingest decimals using the Rust client and update the compatibility matrix in the main README.

Summary by CodeRabbit

• New Features

  • Added decimal datatype support via protocol version 3; requires QuestDB server 9.2.0+.
  • HTTP connections enable decimals automatically; TCP connections require explicit protocol_version=3 configuration.

• Documentation

  • Expanded guidance on decimal ingestion, supported formats (string, bigdecimal, rust_decimal), and protocol/server requirements.

[coderabbitai]

Walkthrough

Documentation updates adding protocol version 3 and detailed Decimal Datatype guidance for QuestDB ingress, including Buffer::column_dec usage, supported decimal types, and server/protocol requirements for HTTP and TCP ingestion.

Changes

Cohort / File(s)	Summary
Protocol versions README.md	Added protocol version 3 row: description "adds decimals" and server compatibility "9.2.0+ (2025-11-14)".
Ingress / Decimal documentation questdb-rs/src/ingress/mod.md	Added "Decimal Datatype" subsection documenting Buffer::column_dec support for decimals via &str, bigdecimal, and rust_decimal; notes that protocol_version 3 is required, HTTP auto-enables with QuestDB 9.2.0+, and TCP requires explicit protocol_version=3; configuration.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~3 minutes

• Documentation-only changes; no API or behavioral edits.
• Review focus: factual accuracy of version numbers and wording.

Poem

🐰 I nibble at digits, precise and small,
Version three hops in to answer the call.
Decimals snug in Buffer's care,
Docs updated with a gentle flair.
A tiny hop for data, a bright new haul ✨

Pre-merge checks and finishing touches

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The PR title accurately summarizes the main changes: adding documentation for decimal datatype and protocol version 3, which matches the README update and the new documentation section in questdb-rs/src/ingress/mod.md.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch rd_decimal_docs

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📜 Recent review details

Configuration used: CodeRabbit UI

Review profile: CHILL

Plan: Pro

📥 Commits

Reviewing files that changed from the base of the PR and between a9003ad and a06006c.

📒 Files selected for processing (1)

• questdb-rs/src/ingress/mod.md (1 hunks)

🚧 Files skipped from review as they are similar to previous changes (1)

• questdb-rs/src/ingress/mod.md

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[coderabbitai]

Actionable comments posted: 1

Caution

Some comments are outside the diff and can’t be posted inline due to platform limitations.

⚠️ Outside diff range comments (1)

questdb-rs/src/ingress/mod.md (1)

273-357: Remove duplicate "Decimal Datatype" section to avoid maintenance burden.

The "Decimal Datatype" documentation appears twice in this file (lines 273–286 and lines 344–357) with nearly identical content. This violates the DRY principle and creates a maintenance hazard: any future updates to decimal support requirements must be applied in two places, increasing the risk of inconsistency.

The first section (lines 273–286) is better positioned because it logically follows the "Array Datatype" section (lines 257–271), establishing a consistent pattern for documenting special datatypes. The second occurrence (lines 344–357) interrupts the flow of optimization tips.

Remove the second occurrence (lines 344–357) and keep only the first section with the grammar fix applied.

📜 Review details

Configuration used: CodeRabbit UI

Review profile: CHILL

Plan: Pro

📥 Commits

Reviewing files that changed from the base of the PR and between 121d7bc and a9003ad.

📒 Files selected for processing (2)

• README.md (1 hunks)
• questdb-rs/src/ingress/mod.md (1 hunks)

🧰 Additional context used

🪛 LanguageTool

questdb-rs/src/ingress/mod.md

[grammar] ~277-~277: Use a hyphen to join words.
Context: ...ng several convenient types: - floating point representation with &str - decim...

(QB_NEW_EN_HYPHEN)

🔇 Additional comments (1)

README.md (1)

58-63: Protocol version table update looks good.

The new row for protocol version 3 is properly formatted, consistent with existing entries, and the date aligns with the server version requirements documented in the complementary Rust ingress documentation.