Move simvascular.github.io documentation to this repository

[mrp089]

Currently, our svFSI- documentation is in a different repository in some sub-directories.

I think having documentation and code in the same place would be good. This way, we can easily check/enforce that the documentation gets updated with the code. We can use our Doxygen build/deploy system and easily add existing (and new) markdown pages to the HTML documentation we're already building. We can link to this svFSI+ website from the main website.

The first step would be sorting through the existing documentation. It looks like part of the documentation isn't built currently, and other parts overlap with the documentation in this repository. I think this would also make sense for the other solvers.

Thoughts, @ktbolt, @MatteoSalvador?