Hide old pkgdb1 retirement messages from the docs.

Things that have 'Legacy' in the test name don't show up in the docs.

These messages are examples from the old pkgdb1.  We want to keep the
tests around for old messages held in datagrepper, but we don't want to
advertise them to users in the docs.
Merge pull request #119 from fedora-infra/feature/legacy-retirement

Hide old pkgdb1 retirement messages from the docs.