Add links to C++ docs GitHub Pages

[jessica-mitchell]

With PR #3597 we will need to tie the Read the docs documentation with the doxygen generated docs. We will also need to heavily modify #3465

Suggestion:

add contextual description of managers and other topics to *.h file and have that documentation live on GitHub pages.

Developer documentation then is split in 2 places

Documentation about C++ code - specifically what the code does, context and meaning should be found in the *.h file
Documenation for contributors/developers about making pull requests, documentation workflows, conventions and coding and reviewer guidelines etc is housed on Read the docs, and the sources are found in doc/htmldoc/developer_space

An exception to this is the model 'userdocs', which we have put in the *.h files but use Sphinx to generate html output. Because this is specific for users but useful for developers, we keep this as is.