Reintroduce support for doc-rooted sites

[chalin]

I've suspected that docsy-only support wasn't working, but I've run out of time to retest the feature. Well, @adlerzei has been confirming that it's broken (thanks!). This is a tracker/meta issue for reintroducing support for it.

Tasks

• Add doc-rooted site example version, update guidance #2563
• Update version+variant menu, add siteGetPage shortcode, add tdVersion struct, and more #2586
• Finalize explanation of how to config doc-rooted sites #2587

Docs

For example see: https://doc-rooted--docsydocs.netlify.app/content/adding-content/#doc-rooted-sites

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Related:

• Top-level section landing pages are always rooted in a docs-only site #2492 - consider docs-only sites for sidebar root default #2496
• Pages missing in the section index of docs-only sites #2499
• Copied from Create a doc-rooted example using the latest version of Docsy #2333, itself copied from Sidenav support for section-as-root in the sidebar #2328:
  • References for toc_root and docsy-only features:
    • Make it easier to create a "docs only" site #368 and https://mostlydocs.netlify.app
    • Support: How to disable landing page section? #173
    • Resources:
      • Repo: https://github.com/LisaFC/justdocs
      • Preview: https://docsyjustdocs.netlify.com/