✨ Add support for rich_help_panel of commands in generated docs

[kinuax]

The generated Markdown docs ignore the division given by rich_help_panel while listing the commands. This is a proposal to support it.

[kinuax]

I added the tests to verify the cases with default (None, "", "Commands") and custom panel titles. All format, linting, and tests pass locally on 3.11. CI only breaks on 3.7 with a syntax error due to the walrus operator (introduced on 3.8). Having that Python 3.7's end of life was almost a year ago and that click dropped 3.7 support 9 months ago, I wonder if this is a good moment to drop its support as well, I can take care of that before considering these changes.

[kinuax]

As clarified at #830, click still supports 3.7 so I have refactored to avoid assignment expressions and pass 3.7 tests. Tests are updated too to verify mixed commands in same app and pass coverage. Ready to be reviewed.