Add comments in setup-build-env/action.yml for where to document prereqs (and the separation between build/run and test).
Add some initial (very basic) text for `test_suites.rst`, listing prereqs and how to run the tests (with subsections for the different optional tests, which is currently docs, functional and unit).
Add sphinx-inline-tabs, use it for tidying up prereq instructions based on OS/platform (mostly helpful in the test suites doc where there are multiple sections split by OS).
Also fixes some single backticks that should be double backtick.
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.