Include remaining repositories' docs in readthedocs site

[aidanfnv]

For the initial documentation move to readthedocs.io, we will only have the standard modules reference and the documents in this repo included directly on the site, with links to other docs like the user guide being links to existing documentation on GitHub pages. These external links should be replaced with links to the docs included as submodules, which may require some changes to the repositories that we are pulling docs from (Slang, SlangPy, etc.)

[aidanfnv]

The readthedocs site now contains the docs from the other repositories that we use for our existing docs pages.