sphinx doc: treat warnings as errors

[tjhei]

inspired by #5370

[tjhei]

/home/docs/checkouts/readthedocs.org/user_builds/aspect-documentation/checkouts/5374/CONTRIBUTING.md:120: WARNING: 'myst' cross-reference target not found: 'doc/modules/changes/' [myst.xref_missing]
/home/docs/checkouts/readthedocs.org/user_builds/aspect-documentation/checkouts/5374/CONTRIBUTING.md:142: WARNING: 'myst' cross-reference target not found: 'doc/modules/' [myst.xref_missing]
/home/docs/checkouts/readthedocs.org/user_builds/aspect-documentation/checkouts/5374/CONTRIBUTING.md:163: WARNING: 'myst' cross-reference target not found: 'AUTHORS.md' [myst.xref_missing]
/home/docs/checkouts/readthedocs.org/user_builds/aspect-documentation/checkouts/5374/CONTRIBUTING.md:167: WARNING: 'myst' cross-reference target not found: 'LICENSE' [myst.xref_missing]

[tjhei]

@gassmoeller Do you know how to fix this? We also want github to render this markdown files and the links correctly, so I assume we can not use the sphinx doc syntax.

[gassmoeller]

Yes I noticed these too in #5370, but couldnt find an easy way to fix them. The problem is that Contributing.md is not in the root path of the sphinx documentation and we use the indirect include in doc/sphinx/user/extending/contributing.md to include it, but this breaks relative links. We could modify Contributing.md to directly link to the files on github (instead of using relative links), it's just not as pretty when you read it in a file editor. Thoughts?

[tjhei]

updated.