[3.12] pythonGH-119054: Add "Reading and writing files" section to pa…

…thlib docs (pythonGH-119524)

Add a dedicated subsection for `open()`, `read_text()`, `read_bytes()`,
`write_text()` and `write_bytes()`..
(cherry picked from commit bd6d4ed)

Co-authored-by: Barney Gale <barney.gale@gmail.com>