Discussions Post on registering studies with Firebase

[RobertGemmaJr]

Rather than including 2 different rulesets we'll only ship with the default rule (registration is done in Prolific) and add a GitHub discussions post about registering studies in Firebase. The post could/should mostly be what's currently in the documentation.

Note that we'll remove the old security rule in version 4.0 and update documentation. This way information for registering studies is not lost but does not clutter the documentation.