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.
If building from read the docs, and the current build is "latest", add `-dev` to the version string.
Requires `YOSYS_VER` to be exported by .readthedocs.yaml.
Images now included relative to the `docs/source` folder instead of the rst file.
Also makes sure to add the updated `yosyshq.css` (which as a sidenote has ended up as `custom.css` in most of the other docs).