DO NOT MERGE: PDB-370 Prepare 1.6.0 PuppetDB documentation for final release

(DO NOT MERGE: we haven't released yet, but are preparing to do so today if possible. I just wanted to give the heads up, feel free to comment and review though :-).

This does two things:

* Removes the warning that this is a pre-release version
* Changes the index so that 1.6 is now marked as 'stable'

Signed-off-by: Ken Barber <ken@bob.sh>