Fix outstanding sphinx warnings;
- Change mycells.lib to use `text` parsing instead of (currently) unsupported `Liberty`.
- Remove unused `troubleshooting.rst`, moving the todo into the index.
Use rtds-action instead of yosys-cmd-ref repo.
Add rtds_action to docs configuration.
Add `.readthedocs.yaml`.
Update `DOCS_USAGE_` make target to be able to use pre-generated executables without forcing a remake.