[CI] Textlint repo .md files, and new link-checker

[chalin]

• Followup to GitHub adjusted to the correct name #3233
• Contributes to [CI] textlint: check repo ./*.md files and track tool fix #3490
• Contributes to CI/infra improvements - 2023/11 #3458
• Ensures that textlint is applied to ./*.md repo files, currently:

  $ ls *.md
  CONTRIBUTING.md README.md

• Adds NPM script _check:links--md, that uses markdown-link-check.
  Note that this script isn't used in CI yet.
• This is the start of an exploration of using a simpler checker for small PRs, and hence aleviating woes with the refache, etc (cf. Refcache Action Issues #3375)