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 (17ab571, 40c012e, 63599eb, e641f48)

[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.

[tammy-baylis-swi]

Thanks again @raajheshkannaa

I should have realized this earlier with my previous comment: the Events API is deprecated and users should use Logging API with event naming instead. This PR does make documentation consistent across the board, but the Events API docs render doesn't include the deprecation information (below screenshot).

What do other @open-telemetry/python-approvers think: include Events API in the docs index, or leave it out?