Sidenav support for section-as-root in the sidebar

[chalin]

Sidebar-root feature

Motivation:

• For deeply nested docs, it would be useful to be able to anchor a section as the root of the left side nav.
• This will also be helpful for versioned docs implemented using subfolders

Tasks

• Add support for sidenav rooted in non-top-level sections
  • sidenav root for "children"
    • [feat] Add support for non top-level section pages as roots in the sidebar #2334
    • Rewrite sidebar_root:true as sidebar_root_for:children #2340
  • "self" sidenav roots
    • Add support for "self" sidebar roots #2341
    • Fix sidebar entry HTML attribute spacing #2342
  • Don't warn when root section declares sidebar_root_for:children #2361
• Resolve the #!forum/docsy-users runtime error - fix via ScrollSpy patch
• UG update
• Changelog entry
• Ensure that this features doesn't unduly increase the processing time to build the sidenav for large sites, esp. if the sidenav is cached
• Ensure that this works with multilingual sites
• Note that this might be able to replace the toc_root feature. This will likely be tackled separately. Yep, see Consider replacing toc_root feature by section-as-root #2410.

References for toc_root and docs-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/

Related:

• Create a doc-rooted example using the latest version of Docsy #2333