Fix documentation

[ptormene]

Fixes #8629

I've fixed some broken cross-references, a malformed table, some indentation issues and various details.
I've added the parameter -W to sphinx-build, to let the build fail in case of any error or warning. Unfortunately I couldn't find any way to let the build produce an exit code different from 0 in case of errors (without -W also errors are ignored, not only warnings).
Sphinx was complaining about language = None and it was falling back to 'en'. I made it explicit. Please @nastasi-oq check if it was not set on purpose.

[micheles]

There was a bug with the exit code of sphinx 6 years ago, but they say it is fixed: sphinx-doc/sphinx#4160 (?)

[ptormene]

I've found also this discussion from April 2022: sphinx-doc/sphinx#10397
So it looks like the problem of the wrong status code was not fixed 6 years ago.

[ptormene]

After

• adding a fallback in case of missing json files for html_theme_options switcher, and
• declaring a the unicode character "⋮" to be mapped by Latex as \vdots,

now I have this green: https://github.com/gem/oq-engine/actions/runs/5133991839/jobs/9237350113
therefore I believe this PR is now ready to be merged.