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

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.

Co-authored-by: Hugo van Kemenade <1324225+hugovk@users.noreply.github.com>