[DOC] Publish the doxygen doc

[marcalff]

Publish the generated doxygen documentation.

• Define where to publish (opentelemetry-cpp pages, or opentelemetry-io)
• Update CI to publish when merging to the main branch
• Retire obsolete doc at https://opentelemetry-cpp.readthedocs.io/en/latest/

[github-actions]

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