Improve docs build

This is a proposal to improve doc build in CI with the following reasoning:

• Adds --keep-going to sphinx-build on original doc to pop up every warning and errors before exiting abnormally, in case of finding errors.
• Also add -W to sphinx-build on original doc to consider warnings as errors, and spot early errors (instead of in shipped docs).
• For translated docs, add --keep-going without -W. I did not add -W because I assumed you don't want to have the pipeline failing for translation errors and GTP coordinators are explicit stating that maintainers should not edit PO files, so better not have it failing on warnings.
• Add sphinx-lint for both original docs and translations to find extra syntax errors, like trailing whitespace. There are many filters, and they can be disabled.