Document how to integrate GitLab chart and CERN PGBouncer chart

Summary

To add documentation for CERN PGBouncer chart integration we need to address several questions:

PGBouncer container

CERN PGBouncer container:

• Consumes PG 13, and we currently expect 14+. Out of the box, this chart is 15. This would need updated.
• The container has not been built for some time, and likely needs updated for security purposes
• As a consideration, have a look at a regularly maintained image from an upstream provider, such as bitnami/pgbouncer.

PGBouncer chart

CERN PGBouncer chart:

• Always attempts to create a Secret, though documentation added in !2973 (closed) says to add this secret manually. These could clash.
• Consumes raw secret content as a Helm value (see .userlist), which is frowned upon. Here is a good article on the topic.
• The chart currently has no license declared or defined in the project it comes from 📜. We require this, hard stop.