doc: cleanup some misc files

doc: cleanup some misc files

* Translate plaintext stub documenting bgpd preprocessor constants for
  attributes into a RST file and include it in the doctree
* Remove the Markdown version of the OSPF-API document, as I've already
  translated it into RST
* Remove the Markdown version of the modules document, as I've already
  translated it into RST
* Rename the various "Building_FRR_*" documents to match standard RST
  file conventions

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