[UG] Navigation page: switch to navbar & sidenav terminology, cleanup markdown

[chalin]

• Fixes Navigation and Menus section confuses concepts of menu vs. navigation #2360
• Switches to using "navbar" and (left) "sidenav" ...
  • ... as replacements for the terms of "top-level menu" and "section menu".
  • The use of "menu" as a qualifier for navs came from the Hugo menu feature used to implement the navs.
  • Why these new terms, because their more intuitive and correspond to website UX terminology. Besides, those are the names we used for the partials that implement those features!
• Along with this rename, I reworked the Navigation page so that it's structure is:
  • Navbar: with subsections that follow the order of navbar items
  • Sidenav
• Converted the tabbed panes in the Navigation page to be more markdown friendly. This allows for a much smoother IDE experience, better syntax highlighting, makes Prettier/Markdownlint igore directives unnecessary, etc.

Preview: https://deploy-preview-2372--docsydocs.netlify.app/docs/adding-content/navigation/

[chalin]

@LisaFC @emckean - I'm starting to think that we should settle on the following terms instead:

• topnav
• sidenav

WDYT?

🤔 Although I'd prefer "topnav" over "navbar" for consistency, it would probably require that we rename the navbar_* configuration params too. I don't want to do that in the scope of this PR. Maybe later, if we agree to do so.

[chalin]

Thanks! I'll address the switch from "Adding" to "Add" in a followup PR.