docs: add missing modules to sphinx documentation build

[raajheshkannaa]

Closes #2573

Summary

• Adds RST files for 5 Python modules that were missing from the sphinx doc build
• Updates toctree entries in index and parent RST files so the new pages are reachable

Modules added

API:

• opentelemetry._events - Events API (EventLogger, EventLoggerProvider)
• opentelemetry.attributes - Attribute validation and utility functions
• opentelemetry.trace.propagation - Trace context propagation helpers (set/get span in context)

SDK:

• opentelemetry.sdk._events - Events SDK implementation (EventLogger, EventLoggerProvider)
• opentelemetry.sdk._logs.export - Log record export (BatchLogRecordProcessor, SimpleLogRecordProcessor, ConsoleLogExporter, InMemoryLogExporter)

Notes

• The existing tox -e docs build has a pre-existing failure (Context cross-reference ambiguity in _logs._internal.LogRecord docstring) that is unrelated to this change and present on main
• All new RST files follow the existing patterns used throughout docs/api/ and docs/sdk/
• This is a partial fix for Some documentation not built into sphinx docs #2573. The issue also mentions automating RST generation, which could be a separate follow-up

Test plan

• Ran tox -e docs locally; all new modules are picked up by sphinx-build (visible in the "reading sources" and "highlighting module code" output)
• Verified the only build warnings are pre-existing (not introduced by this change)

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: raajheshkannaa / name: Raajhesh Kannaa Chidambaram (57e35a2, c7345dc, d93aa2b, f45c028)

[tammy-baylis-swi]

Thanks @raajheshkannaa . Please could you sign the CLA as per contributing guidelines.

[tammy-baylis-swi]

Thanks! Next please run tox -e docs to fix some issues:

/home/runner/work/opentelemetry-python/opentelemetry-python/opentelemetry-sdk/src/opentelemetry/sdk/_events/__init__.py:docstring of opentelemetry.sdk._events.EventLogger.emit:1:py:class reference target not found: Event

[raajheshkannaa]

Rebased on main and ran tox -e docs. The Event reference was already covered by nitpick_ignore from the earlier commit, but the rebase surfaced a new unresolved reference for Token (from contextvars). Added that to nitpick_ignore as well. Docs build passes clean now.