[docs] Move gen/build_arguments.md to //docs

The old gen/build_arguments.md files from the pre-fuchsia.git repos
have not been updated in a long time.  The docs-roller bot now
updates //docs/gen/build_arguments.md instead.  But since that file
didn't exist, bot runs have not been updating *any* file for the
Fuchsia GN build arguments documentation, only the new Zircon file.

Remove the defunct peridot/ file and move the garnet/ file to
//docs where the next bot run should update it properly.

Change-Id: I6430b4af4ca796fa12ae366ce899c1c3afc0856b