[3.12] GH-119054: Add "Reading directories" section to pathlib docs (GH-119956) (#120184)

Add a dedicated subsection for `Path.iterdir()`-related methods,
specifically `iterdir()`, `glob()`, `rglob()` and `walk()`.

(cherry picked from commit 14e1506a6d7056c38fbbc0797268dcf783f91243)

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>