doc: improve table-of-contents organization

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

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

Signed-off-by: Quentin Young <qlyoung@qlyoung.net>

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