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
• ...

Related

• Pages missing in the section index of docs-only sites #2499
• Top-level section landing pages are always rooted in a docs-only site #2492

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Copied from #2333, itself copied from #2328:

• References for toc_root and docsy-only features:
  • Make it easier to create a "docs only" site #368
  • Support: How to disable landing page section? #173
  • Resources:
    • Repo: https://github.com/LisaFC/justdocs
    • Preview: https://docsyjustdocs.netlify.com/