doc: improve table-of-contents organization

[qlyoung]

The current TOC organization is not really following Sphinx best practices and is resulting in a jumble of articles showing up in the sidebar.

Result of these changes is viewable here: https://qlyoung.net/frr-docs/
Compare to: https://docs.frrouting.org/en/latest/

This change primarily organizes existing articles into three major sections:

• Introduction
  • Contains system requirements, architecture & design, installation, basic setup
• Basics
  • Covers basic commands, concepts, and some random things that don't fit elsewhere
• Protocols
  • Contains all protocol documentation, and other miscellaneous daemon docs such as those on Zebra, watchfrr, mgmtd, etc.

The appendix has been left as is, but the TOC now has a caption which has the effect of adding a section separator in the nav sidebar.

In order to make the new structure make sense:

• Some content has been lifted up from the "Overview" page into the index page
• Most content has been pushed down from the "Overview" page into the "About" page (new)
• BFD's page is now titled "BFD" for consistencty; it was the only one that had the full protocol name written out in the title

And a couple drivebys:

• BFD's intro description paragraph was rewritten to make more sense
• Old language stating that we publish platform packages on the Github releases page was removed
• References to source building instructions were consolidated into that section

[riw777]

looks good

[qlyoung]

ci:rerun

[ton31337]

LGTM