CI: Sphinx virtual build for docs.shader-slang.org #152
Description
We currently do not do any pre-submit testing for the docs website, which means that issues in Sphinx files (such as docs/conf.py) can be merged even if they will break the ReadTheDocs build.
The Sphinx build run by ReadTheDocs can be run locally without deployment, so we can and should have a check to make sure that at least builds correctly before merging PRs.