Extract _generate_directives() from Documenter._generate()

[AA-Turner]

References

• Make autodoc more maintainable #13751

[bsipocz]

This currently causes downstream CI failure in sphinx-automodapi as we use get_documenter from autosummary. This is just a fyi as I haven't had a chance yet to dive into the code to see how complicated the refactoring on our end should be.

[AA-Turner]

Thanks @bsipocz. I plan to add some transitional layer from the class/mixin-based approach to the pure functional approach, so this will definitely be resolved before any release, but it's always useful to know about potential impact.

A