Initial effort to bring pex_config to the Sphinx-based documentation system.
- Adding Sphinx documentation infrastructure for packages.
- Convert docstrings in lsst.pex.config to Numpydoc format to generate an API reference.
- Port existing user guide content from pex_config to reStructuredText (though fully converting this content into topic-based documentation is not in this scope).
This is related to
DM-11253 for pipe_base.