Adds sphinx docs

I made sure that the setup supports both markdown and restructured text (rst). Rst is needed to use the fancy sphinx directives like table of contents and stuff. But inner pages can just use markdown that everyone is comfortable with.

There is a readme in docs/README.md that explains how to build the docs.

make docs/build

or

make docs_image
make docs_shell
# inside the shell
make html

I did nothing else except just confirm that everything works. I figure actually writing the docs would be a separate MR to keep this one reviewable.