Enable numpydoc pre-commit checks subpackage by subpackage #20353

This is a long-term follow-up of #15386 and #20284 . There is a numpydoc pre-commit enable but most subpackages are ignored. The goal is to have a series of small(ish) PRs that is scoped within a single subpackage. If a subpackage itself is large, it can be further split into multiple PRs in some logical way (what would Spock do?).

In scope: API docstring that we render with Sphinx.

Out of scope: Bundled files (extern), test files, setup files, etc.