docs: Document manual session tracking for non-WSGI/ASGI projects

[DawnofGenX]

Summary

Closes #1461. Documents how to add manual session tracking for projects that are not served over WSGI/ASGI (CLIs, workers, daemons), which is what the issue asks for.

The original issue snippet uses the deprecated Hub + auto_session_tracking(hub) API. This documents the current public APIs instead:

• sentry_sdk.sessions.track_session(scope, session_mode=...) context manager (the pattern already used internally by the WSGI/ASGI/aiohttp integrations), used together with the public isolation_scope() context manager.
• sentry_sdk.start_session() / sentry_sdk.end_session() for explicit control.

Changes: a new "Session Tracking" section in docs/api.rst with prose + two runnable-style examples, plus autofunction directives for track_session, start_session, and end_session (these previously had no home in the docs at all).

Context from the thread: @sentrivana already said "go ahead and open a PR" in #1461 (comment).

Test plan

• rm -rf docs/_build && uv run --group docs sphinx-build -vv -W -b html docs/ docs/_build — same command as the docs job in .github/workflows/ci.yml, exits 0 (warnings would be errors with -W):

build succeeded.

The HTML pages are in docs/_build.

• Verified the documented APIs exist and match their signatures on master (sentry_sdk/api.py start_session/end_session, sentry_sdk/sessions.py track_session, sentry_sdk/__init__.py exports).

Fixes #1461

[alexander-alderman-webb]

Going to close this because we don't have capacity to review the 5 PRs you've opened.
If you'd like to contribute dedicate your time towards making one good PR. Thanks.