WIP: Use Sphinx Verilog domain

[imphil]

The Sphinx Verilog domain enables semantic description and
cross-referencing of Verilog features, such as parameters, ports,
modules, etc.

The Sphinx Verilog domain is rather experimental currently. This PR
serves as starting point, but will need to align with improvements to
the Sphinx Verilog domain before it can be merged.

Dependent issues:

• Add a "currentmodule" directive to avoid nesting SymbiFlow/sphinx-verilog-domain#23
• Add support for parsing enum parameter types SymbiFlow/sphinx-verilog-domain#22

CC @mglb @rw1nkler @mithro

[imphil]

I haven't seen any progress on the blockers for this PR, closing until upstream catches up.