Check stylistic and formal issues in .rst and .py files in the documentation
Description
Sphinx Lint should:
- be reasonably fast so it's comfortable to use as a linter in your editor.
- be usable on a single file.
- not give any false positives (probably a utopia, but let's try).
- not spend too much effort finding errors that sphinx-build already finds (or
can easily find).
- focus on finding errors that are not visible to sphinx-build.
