[CI] Add Doxygen validation workflow

[srgbtl]

Fixes #1647

Changes

Added a new doxygen-tidy workflow that runs on every PR and push to main to validate Doxygen comments and ensure documentation quality is maintained. Also fixed all Doxygen warnings found so far.

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.99%. Comparing base (ad8b961) to head (467d881).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3882   +/-   ##
=======================================
  Coverage   89.99%   89.99%           
=======================================
  Files         225      225           
  Lines        7170     7170           
=======================================
  Hits         6452     6452           
  Misses        718      718           

Files with missing lines	Coverage Δ
api/include/opentelemetry/logs/logger.h	62.86% <ø> (ø)
...pentelemetry/sdk/logs/batch_log_record_processor.h	100.00% <ø> (ø)
sdk/include/opentelemetry/sdk/resource/resource.h	100.00% <ø> (ø)
...ude/opentelemetry/sdk/trace/batch_span_processor.h	100.00% <ø> (ø)

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