Add changelog entry for PR #3792 (FastAPI documentation improvements)

[Copilot]

This PR adds the missing changelog entry for PR open-telemetry#3792 to the main CHANGELOG.md file.

Background

PR open-telemetry#3792 added comprehensive documentation to the FastAPI instrumentation README, including:

• Environment variables configuration (OTEL_EXPORTER_OTLP_ENDPOINT, OTEL_SERVICE_NAME, etc.)
• Trace propagation details for HTTP requests
• WebSocket support limitations
• Logging integration instructions using opentelemetry-instrumentation-logging
• Links to related PyPI packages

While the PR description indicated that "Changelogs have been updated", the entry was not present in the main CHANGELOG.md file under the "Unreleased" section.

Changes

Added a single changelog entry under the "### Added" section of "## Unreleased":

- `opentelemetry-instrumentation-fastapi`: Add documentation section on environment variables, trace propagation, websockets, logging, and package links
  ([#3792](https://github.com/open-telemetry/opentelemetry-python-contrib/pull/3792))

This entry follows the existing changelog format and properly documents the documentation improvements made to the FastAPI instrumentation package.

Original prompt

On CHANGELOG.md, Add the changelog in PR open-telemetry#3792 to this CHANGELOG.md file.

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