[CI] textlint: check repo ./*.md files and track tool fix

[chalin]

• Context: part of the fixes of GitHub adjusted to the correct name #3233 should have been detected by textlint in CI, but they weren't
• I've filed an issue with a minimally reproducible test case, see:
  • Mis-capitalized word not detected at end of line in markdown file textlint/textlint#1281
  • Mis-capitalized word not detected at end of line in markdown file sapegin/textlint-rule-terminology#71
• I'm creating this issue so that we can follow up on the textlint fix if/when it lands. I suspect that with the fix, the tool will report more issues (at least that's what I've seen in some quick tests).

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• Ensure that top-level repo markdown files also get checked (so that our community doesn't have to manually fix issues, like in GitHub adjusted to the correct name #3233) - [CI] Textlint repo .md files, and new link-checker #3491
• Upgrade textlint, run it and address issues; once the following is addressed -
  [CI] Upgrade textlint-rule-terminology to bring in patch #3504:
  • Mis-capitalized word not detected at end of line in markdown file sapegin/textlint-rule-terminology#71
  • Fix word detection at end of line sapegin/textlint-rule-terminology#72