GH-110109: pathlib docs: bring `from_uri()` and `as_uri()` together. (#110312)

This is a very soft deprecation of `PurePath.as_uri()`. We instead document
it as a `Path` method, and add a couple of sentences mentioning that it's
also available in `PurePath`.

Co-authored-by: Adam Turner <9087854+AA-Turner@users.noreply.github.com>