Add readthedocs on the repository

[mithro]

Either Sphinx or Mkdocs for generation documentation from the repo contents automatically and publishing at something like riscv-cfu.readthedocs.io

With Sphinx we can use symbolator and my new sphinxcontrib-verilog-diagrams extension too.

[mithro]

• https://docs.readthedocs.io/en/stable/intro/getting-started-with-sphinx.html
• https://sphinxcontrib-verilog-diagrams.readthedocs.io/en/latest/

[mithro]

Pretty good example to steal from is at https://github.com/SymbiFlow/sphinxcontrib-verilog-diagrams

[grayresearch]

We have transitioned to asciidoc, wavedrom, etc.