doc: massive cleanup of bgp.rst

doc: massive cleanup of bgp.rst

This doc needed (and still needs) a lot of love.

* Reorganize doc into logical sections
* Clean up lots of grammatical mistakes and misspellings
* Clean up lots of bad formatting
* Remove non-existent configuration options and commands
* Mark deprecated configuration options as deprecated
* Fix all cross-references to follow a standard format
* Fix capitalization on section and subsection titles
* Fix section and subsection level underlines

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