Create a just-docs example using the latest version of Docsy

[chalin]

(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/