Docs: New swipe-to-refresh training guide.

Doc-in-progress inherited from xoogler's (unfinished) CL
http://ag/427505 . See comments for staging location.

Patch set 1 is the version inherited from the xoogler; PS2 is after
my edits. Per a conversation with CB, I've removed the "ListFragment"
doc, and just put in a note about that topic in the
"add-swipe-refresh" doc.

Change-Id: Id1a1294b15c5a7363f650ea13237948b0effbc40