Docs jobs should not skip on concurrent jobs (in case a non-docs job is already
running, such as when a commit has been tagged for docs-preview). However, a
successful `test-verific` can allow for a future docs job to skip testing and go
straight to the preview.
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.