[3.13] pythonGH-119054: Add "Reading directories" section to pathlib …

…docs (pythonGH-119956)

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

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>.
(cherry picked from commit 14e1506)

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