External link checking + project pages

[chalin]

• Fixes [CI] NPM script check:links--md no longer working #2113
• Fixes [CI] Publish CHANGELOG and other top-level *.md files on the Docsy site and drop markdown-link-check #1781
• Adds support for external link checking
• Makes checking of external links (in addition to local paths) the default
• Temporarily ignores the docs section. We'll incrementally consider the section as we build the refcache
• Adds a new Project section to the site
  • Publishes the project's top-level .md files, such as the CHANGELOG, in this new section
• Drops markdown-link-check since we now use htmltext (which caches external links) to check all links

Preview:

• https://deploy-preview-2120--docsydocs.netlify.app/project/changelog/
• https://deploy-preview-2120--docsydocs.netlify.app/project/

Note: with the refcache built, it takes less than a second to check all (including external) links on the site.

Followup:

• Rework contributing pages so that they are DRY #2119
• Document options for breadcrumbs #810
• Can we get the mounted pages to display in the left nav?