Fail in CI if docs build contains logs about broken internal links

[inglesp]

We should use CI to prevent changes to the docs from introducing broken internal links.

There are two ways of introducing broken internal links. CI should fail if either of these are detected.

The first is by linking to a page that doesn't exist. When this happens, we see something like the following in the output:

WARNING -  Doc file 'explanation/backend-tables.md' contains a link
           '../tutorial/writing-a-dataset-definition/index.md', but the target
           'tutorial/writing-a-dataset-definition/index.md' is not found among documentation
           files.

The second is by not linking to a .md file. When this happens, we see something like the following in the output:

INFO    -  Doc file 'tutorial/more-complex-transformations/index.md' contains an unrecognized
           relative link '../../how-to/codelists/', it was left as is. Did you mean
           '../../how-to/codelists.md'?

It may be possible to configure mkdocs to return non-zero if a broken link is detected. Otherwise, we'll have to check the error messages.

A broken external link should not cause our main CI to fail, because external links may break for reasons unrelated to a PR. Instead, broken external links are checked by the weekly link checker.