Document use of bandersnatch for pypi mirroring

We use bandersnatch to mirror pypi now and that should be documented.
Add all the usual header information with quick links to useful places
like local config and upstream bug trackers. Also, note how we run
bandersnatch and details on detecting and correcting stale packages on
pypi preventing local syncs.

Change-Id: I98db04c19c427335a4786a7923982fabcf57051b
Merge "Document use of bandersnatch for pypi mirroring"