Add documentation on models affecting or being affected by signals

[vickyszuchin]

Issue description

We have code, especially for Users and Contacts that makes changes by using django signals. However this is very hard for new developers to find out as it's very buried in the code. The goal here is to make sure a new developer would easily know that we 1.) use signals. 2) where we use it. 3) how we use it. 4) when looking at the given model affected by the signal its clearly documented what is relying on the signal.

Acceptance criteria

• Add documentation in our developer readme (or a similiar readme) to explain that we are using signals on some models (answer why, how and where).
• On models affecting or being affected by signals, add documentation on what is being changed by a signal and when (and any other useful info deemed worthwhile to add)

Additional context

Discussed in engineer huddle on 4/8/2024. See notes.

Links to other issues

No response

[vickyszuchin]

Removed "pulled in" label since this is planned for the upcoming sprint (starting on 5/8).