PuppetDB: New low catalog troubleshooting guide

This adds the new guide into the index for master, in ancticipation for a
future release.

Signed-off-by: Ken Barber <ken@bob.sh>
Merge pull request #246 from kbarber/maint/master/puppetdb_trouble_low_catalog_dupe

PuppetDB: New low catalog troubleshooting guide