Add Docker documentation.

Also refactor VM stuff a little, and add a small Xen section.
8 jobs for docker-docs in 2 minutes and 32 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Build
passed #2337
apt

00:00:37

passed #2349
cloudlab

00:01:35

passed #2350
emulab

00:00:38

passed #2340
phantomnet

00:00:36

failed #2338
cloudlab

00:00:34

failed #2339
emulab

00:00:36

 
  Deploy
passed #2341
pages

00:00:19

passed pages:deploy

00:00:01