Add utility to find code blocks in RST files

[felixfontein]

Generalizes the code from ansible/ansible-documentation#2385 so it can be used in other contexts.

(I'm planning to add YAML validation to code blocks in extra docs to antsibull-nox.)

To do:

• Polish.
• Find row/column in tables.
• Maybe figure out how to undo directive registration, since this is a rather destructive process. (Might not be possible without accessing docutils internals, but 🤷)

[felixfontein]

I think this is a good first version. Finding the actual code inside tables is something we can add later. And unregistering directives resp. registering them only temporary doesn't look very realistic, when looking at docutils' code...