[feat] Add support for non top-level section pages as roots in the sidebar

[chalin]

• Contributes to Sidenav support for section-as-root in the sidebar #2328
• Adds support for non-top-level sidebar rooted sections
• Reformats the sidebar related layouts for better readability -- there were some very long lines in there!
• Sets the UG section on "Content and Customization" as a section root, to illustrate the feature
• I'll make the changes to the UG etc in a followup PR. I'd like to test this core functionality on other sites first.
• Includes dev notes under (the new folder) /tasks
• When this feature is disabled, there are no changes to the generated UG pages

Preview: https://deploy-preview-2334--docsydocs.netlify.app/docs/adding-content/content/

Screenshots

C & C section root:

This is as it was before: