readthedocs: Add doxygen-generated docs for sdk, and placeholder to add more sdk docs

[lalitb]

Changes

This PR adds

• Doxygen generated doc for SDK. The current doc has source-tree documentation for API. This PR adds source-tree documentation for SDK.
• Create a placeholder for adding more doc write-ups for SDK and subsequently also exporters. ( will create a follow-up PR for that)

This is how it looks with this PR: https://labhas-opentelemetry-cpp.readthedocs.io/en/latest/
Existing doc: https://opentelemetry-cpp.readthedocs.io/en/latest/

( note: themes are different for now. Finally, we would be using the existing theme ).

More technical details:
The breathe plugin doesn't seem to handle multiple projects well, so the complete opentelemetry doc is as a single project now. Also, the sphinx theme is now changed to sphinx_rtd_theme. Will revert it back to furo theme in subsequent PR, as this was causing some issues.

[codecov]

Codecov Report

Merging #902 (2de6bb6) into main (be32bde) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #902   +/-   ##
=======================================
  Coverage   95.40%   95.40%           
=======================================
  Files         158      158           
  Lines        6717     6717           
=======================================
  Hits         6408     6408           
  Misses        309      309