Use Sphinx config to discover directives

[JulienPalard]

An idea of @jean-abou-samra, see: sphinx-doc/sphinx#10143 (comment)

I like the idea: it should lead to less false positives, which is very important. It could even help spot typos in directive names!

As long as it's still fast to check a single file, so it can be used as a linter in an editor, I don't want rstlint to become slower than make suspicious from cpython, on my machine, for a full cpython/Doc/ check:

• sphinx-lint takes 5.7s
• make suspicious takes from 36s to 1m22s (depends on the cache)

[jeanas]

Not sure if I'll have the time soon, but I'm basically interested in working on this.