[3.13] GH-119054: Add "Expanding and resolving paths" section to pathlib docs. (GH-120970) (#121155)

GH-119054: Add "Expanding and resolving paths" section to pathlib docs. (GH-120970)

Add dedicated subsection for `home()`, `expanduser()`, `cwd()`,
`absolute()`, `resolve()` and `readlink()`. The position of this section
keeps all the `Path` constructors (`Path()`, `Path.from_uri()`,
`Path.home()` and `Path.cwd()`) near the top. Within the section, closely
related methods are kept adjacent. Specifically:

- `home()` and `expanduser()` (the former calls the latter)
- `cwd()` and `absolute()` (the former calls the latter)
- `absolute()` and `resolve()` (both make paths absolute)
- `resolve()` and `readlink()` (both read symlink targets)
- Ditto `cwd()` and `absolute()`
- Ditto `absolute()` and `resolve()`

The "Other methods" section is removed.

(cherry picked from commit d6d8707ff217f211f3a2e48084cc0ddfa41efc4d)

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