[3.12] GH-119054: Add "Reading and writing files" section to pathlib docs (GH-119524) (#119955)

Add a dedicated subsection for `open()`, `read_text()`, `read_bytes()`,
`write_text()` and `write_bytes()`.

(cherry picked from commit bd6d4ed6454378e48dab06f50a9be0bae6baa3a2)