DOC: Implement script to validate list indentation in docs
In #21518 it's been identified that several lists in the documentation don't follow the restructuredText standard (no indentation for top-level lists, 4 space indentation for sublists).

In that issue, (hopefully) all the formatting has been fixed manually. But it'd be useful to have a script that validates all the documentation pages, and makes sure no wrong formatting exists. Adding this script to `lint.sh` will also prevent that no lists with the wrong formatting are added in the future.