Remove rst-style underlining from newly-introduced markdown, add regression check

[peterbarker]

Summary

Fixes regression in markdown sanity, adds a regression check

Classification & Testing (check all that apply and add your own)

• Checked by a human programmer
• Non-functional change
• No-binary change
• Infrastructure change (e.g. unit tests, helper scripts)
• Automated test(s) verify changes (e.g. unit test, autotest)
• Tested manually, description below (e.g. SITL)
• Tested on hardware
• Logs attached
• Logs available on request

When bug is re-introduced;

Checking 2 commit(s) since master...

✓ No merge commits.
✓ No fixup! commits.
✓ All commit messages have well-formed subsystem tags.
✓ All commit subject lines within 160 characters.
✓ No unacceptable author emails.
✓ All submodule updates are isolated in their own commits.
~ markdownlint-cli2 not installed.
✗ Setext/RST underline-style headings found in markdown file(s):
         libraries/AP_HAL_ChibiOS/hwdef/PrincipIoTH7Pi/README.md:10: '============'
       Use ATX-style headings (# prefix) instead.

1 check(s) failed.

Description

Fixes new markdown to remove udnerlines (it's the only markdown file in-tree which uses them)

Makes sure they can't sneak in again!

[Hwurzburg]

@peterbarker does the latest change stop checking merge commits on wiki PRs? master merge commits in the wiki is a pain I would like to reject and force rebases

[peterbarker]

@peterbarker does the latest change stop checking merge commits on wiki PRs? master merge commits in the wiki is a pain I would like to reject and force rebases

I answered Henry elsewhere; the short answer is no.

And I created a branch against the Wiki to add check_branch_conventions.py while I was at it :-)