Create a symbiflow-sphinx-docs-common repository

It feels like there is a lot of duplication between all the symbiflow (and some non-symbiflow repositories like the Fomu workshop) around getting a nice sphinx setup.

It would be good if there was a common repository that could be shared between everything. This could also be a good place to use `git subtrees` (rather than `git submodules`) to pull the module into the dependent repositories.
- https://www.atlassian.com/git/tutorials/git-subtree
- https://blog.developer.atlassian.com/the-power-of-git-subtree/

The repository should include;
- Setting up the conda environment.
- Pulling in sphinx modules to be used
- [ ] The material design theme - https://github.com/SymbiFlow/sphinx_materialdesign_theme
- [ ] https://github.com/SymbiFlow/sphinxcontrib-verilog-diagrams
- [ ] https://github.com/mithro/sphinxcontrib-session
- [ ] Potentially other things at https://github.com/mithro/sphinx-contrib-mithro
- Build on readthedocs
- Others?

We should also decide if we want to use `m2r` rather than the existing rather hacky https://github.com/SymbiFlow/sphinxcontrib-markdown-symlinks and recommonmark.