Improve cli help somewhat, creating an Experimental group

[wyattscarpenter]

This improves a couple of things relating to the cli help. It changes some verbiage and also invents the Experimental group. Having an experimental group fixes the awkward situation where the --enable-incomplete-feature would just float randomly at the top of the cli help, not in a group. For this Experimental group, I ignored --allow-redefinition-new, because it's better where it is currently, right next to --allow-redefinition.

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Old description of PR, no longer relevant, but referenced later in discussion.

This PR rearranges a couple of command line flag categorizations to make more sense and not leave any uncategorized — which is debatable, if someone really liked how the current top of mypy --help is

options:
  --enable-incomplete-feature {InlineTypedDict,PreciseTupleTypes}
                            Enable support of incomplete/experimental features for early preview

Optional arguments:
  -h, --help                Show this help message and exit
  -v, --verbose             More verbose messages
  -V, --version             Show program's version number and exit
  -O FORMAT, --output FORMAT
                            Set a custom output format

Config file:

instead of

Utility arguments:
  -h, --help                Show this help message and exit
  -v, --verbose             More verbose messages
  -V, --version             Show program's version number and exit
  -O FORMAT, --output FORMAT
                            Set a custom output format (choices: {'json'})

Config file:

This PR also programmatically enforces the cli help style guide (from which it removes the advice about links that seemed misguided).

This PR builds on #19062, to save time fixing merge conflicts later, and so you should probably merge that first!

This PR has a long history because it used to contain many other changes, but that history can be ignored as I broke most of those changes out into different PRs.

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅