Move RFC to google doc

Summary:
This RFC is what people are referencing but is constantly getting out-of-date due to modifier API changes.

Move the RFC itself (the API page) to a google doc so it stays more up-to-date and enables comments.
Deleted the use cases page because it's difficult to keep this up-to-date, and it's much simpler to add back a page of real examples of modifiers instead.

For staticdocs, I changed existing RFC page to be internal-only and point to the google doc.

Unfortunately, this does mean that OSS no longer gets to see the RFC, but I'm fixing that in next diff by publishing it as a PDF instead.

Reviewed By: Will-MingLun-Li

Differential Revision: D64274616

fbshipit-source-id: 3eee097c7fc728a68464a60324b72655336cb738

docs/rfcs/attr-metadata.md

@@ -26,7 +26,7 @@ target graph.
 - testinfra wants to use `PACKAGE` values to mark a set of folders to a logical
   larger project
 - it is likely that per-target `metadata` attribute should be used in
-  [configuration factory function](cfg-modifiers/api.md).
+  configuration factory function.
 - TD wants to declare CI trigger jobs per-target or per-package, and this logic
   is to be specified in `BUCK` or `PACKAGE` files — as metadata