Document how the Registration system currently works

[dunkOnIT]

A documented understanding of how the Registration system works is a pre-requisite to porting it to React.

This is currently being worked on in a (WST-only) Google Doc.

[notrichardpeng]

I'll be logging my progress here, but pls feel free to edit or comment on the doc as well: added some stuff about deleting registration and selecting events

[notrichardpeng]

Added info about admin editing registrations

[notrichardpeng]

Let me know if I missed anything...

[gregorbg]

Thanks for your work so far! I think it's fine, but there is no objective standard to judge this by (the reason why you created this documentation is that a similar standard did not exist before). We can start the (re-)implementation based on your work, and then extend the document when needed.

[dunkOnIT]

Thanks for your work on this Richard! It has been noted in this issue