(PDB-784) Final release changes for PDB 2.2.0

This pushes PuppetDB 2.2.0 to be the primary documentation, and fixes the index
so it references 2.2 instead of master (my mistake from last commit).

Signed-off-by: Ken Barber <ken@bob.sh>
Merge pull request #375 from kbarber/ticket/master/pdb-784-pdb-release-220

(PDB-784) Final release changes for PDB 2.2.0