[3.12] GH-119054: Add "Creating files and directories" section to pathlib docs. (GH-120186) (#120464)

Add dedicated subsection for `pathlib.Path.touch()`, `mkdir()`,
`symlink_to()` and `hardlink_to()`. Also note that `open()`, `write_text()`
and `write_bytes()` are often used to create files.

(cherry picked from commit c2d810b6d4deeea530648a8d0983e3a2adf6c942)