Host the documentation on a webpage

[joanibal]

Use Readthedocs.com or an alternative service to provide a place to view the documentation without needing to build it locally. This will also serve as means to publicize the code.

[kejacobson]

Github actions now generate docs upon merges into main branch and push to gh-pages branch to host on github pages: https://openmdao.github.io/mphys/

Link added to repo's README