docs: Begin documenting ISL
I've got a bunch of supplamentary documentation for ISL that I'd like to get in the tree. I'd really like it to be part of the sphinx-generated documentation. However, I'd also really like to be able to pull as much of it from isl.h as possible. To that end, I've attempted to add doxygen+sphinx support via breathe here. I'm happy to take suggestions for how to do this and/or get help integrating it with GitLab CI and readthedocs.
We've also got some NIR docs that it'd be great to eventually incorporate similarly.