Document push key acceptance.

Gerrit replication calls out to ssh which requires host keys to be
accepted in the relevant user account. Document this to ease initial
deployment.

Change-Id: I03b8f1c39a4562cc480b3ef612cb3306b1445814
Improve Zuul docs.

The list of pipelines (duplicated with the layout.yaml) was incomplete.

No node size was described, and the need (or lack thereof) for backups
wasn't covered.

Change-Id: Ia20b610e7c51184239910d6b6e902390593fcaff
Improve Jenkins documentation.

The initial setup was not documented at all.

Change-Id: I074eabac958b77ecf3a333c9ea9beabd539b2f9e
Merge "Document push key acceptance."
Merge "Improve Zuul docs."
Merge "Improve Jenkins documentation."