docs(fastapi): document env vars, trace propagation, websockets, and logging

[rut0607]

Fixes #3301

This PR adds missing documentation to the FastAPI instrumentation page, covering gaps noted from an outside-Python-ecosystem perspective:

• Clarifies that general SDK env vars (OTEL_EXPORTER_OTLP_ENDPOINT, OTEL_SERVICE_NAME, etc.) are configured separately from FastAPI-specific vars, with a link to the SDK docs
• Documents that trace context propagation works automatically for incoming requests via the ASGI middleware and configured propagators
• Confirms and documents WebSocket support (verified against the existing test suite in test_fastapi_instrumentation.py)
• Documents how to enable log-trace correlation via the separate logging instrumentation package and OTEL_PYTHON_LOG_CORRELATION
• Adds a direct PyPI link for discoverability

A changelog fragment is included at .changelog/PLACEHOLDER.added (will rename to match the PR number once assigned).

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: rut0607 / name: Rut Vaghani (0e0c122, 3c35e5b)