ruff_python_formatter: move docstring handling to a submodule (astral…

…-sh#8861)

This turns `string` into a parent module with a `docstring` sub-module.
I arranged things this way because there are parts of the `string`
module that the `docstring` module wants to know about (such as a
`NormalizedString`). The alternative I think would be to make
`docstring` a sibling module and expose more of `string`'s internals.

I think I overall like this change because it gives docstring handling a
bit more room to breath. It has grown quite a bit with the addition of
code snippet formatting.

[This was suggested by
@charliermarsh.](astral-sh#8811 (comment))