[3.12] gh-126071: Improve formatting of the argparse documentation (GH-126073) (GH-126174)

* Use appropriate roles for ArgumentParser, Action, etc.
* Remove superfluous repeated links.
* Explicitly document signatures and add index entries for some methods
  and classes.
* Make it more clear that some parameters are keyword-only.
* Fix some minor errors.
(cherry picked from commit 2ab377a47c8290f8bf52c8ffb5d7fc4c45452611)