Disable doxygen / breathe / exhale generation of C++ API docs (#80451)
Fixes #79992
This PR:
* Removes doxygen / breathe / exhale configuration from the Sphinx config in `source/conf.py` so it no longer runs
* Maintains the human-generated content describing API usage in the various .rst files
* Exception: `library.rst` is removed, as its main purpose is linking to API docs
* Removes all links to the generated API docs from the human-generated content
The build is nearly instantaneous now and should be much less memory intensive as well.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/80451
Approved by: https://github.com/suo
