Add sphinx documentation and readthedocs generation

[mithro]

Sphinx (https://www.sphinx-doc.org/en/master/) is the used by a large number of the existing Python and open source EDA ecosystem (for example the Amaranth's own documentation uses Sphinx, the Google open source PDK's use it like SKY130 and GF180MCU, the F4PGA documentation -- even the Yosys documentation seems to be converting too) and it would be great to see this used here!

See also Teo's documentation @ https://github.com/tdene/synth_opt_adders/tree/main/docs -- There are also examples are http://cfu-playground.rtfd.io/, https://workshop.fomu.im/en/latest/

Other useful extensions for hardware are;

• https://github.com/SymbiFlow/sphinxcontrib-hdl-diagrams
• https://github.com/bavovanachte/sphinx-wavedrom
• https://github.com/SymbiFlow/sphinx-verilog-domain

[mithro]

@umarcor probably has suggestions and thoughts as well!

[mithro]

@antonblanchard - Would you be interested in having this setup?