Eventually the infra/config/generated directory will contain
many configs generated from starlark code.

* Remove /infra/ from .gitignore
* Document the new directory in layout.md
* Add a few infra folks to OWNERS
  * garymm: working on starlark configs for all infra
  * nmulcahey: has been reviewing recipe proto API changes.

Bug: IN-1102 #comment
Test: CQ
Change-Id: Ie231f5cab1856f9280e283006e3acc12751385d2