Update ADK doc according to issue #1048 - 8

[adk-bot]

In docs/tools-custom/function-tools.md, add a new section.

Current state:
The documentation explains how to define function signatures.

Proposed Change:
Add a new section about the JSON schema-based function declaration.

## Simplified Tool Declaration with JSON Schema (Experimental)

ADK now offers a simplified way to declare function tools by generating a JSON schema from your function's signature using Pydantic. This feature can be enabled via a feature flag.

When enabled, `build_function_declaration` will automatically generate a JSON schema for your function's parameters, which simplifies the declaration process and provides more accurate type information to the LLM.

This feature is currently experimental and can be enabled by setting the `ADK_ENABLE_JSON_SCHEMA_FOR_FUNC_DECL` environment variable to `true`.