feat(docs) Add docs for deletions subsystem

[markstory]

I thought these docs were a little too low-level and specific to be in develop.sentry.io and decided to expand the existing module docstring instead.

[wedamija]

This looks good.

One thing that would be good to figure out as a backend team would be how to remind people to add things to deletions. I forget to do this all the time, and I'm sure many people aren't aware of it at all.

I wonder if there's a way we can automate it somehow. Like all models need to be explicitly added to some list to mark off that deletions have at least been considered

[markstory]

One thing that would be good to figure out as a backend team would be how to remind people to add things to deletions. I forget to do this all the time, and I'm sure many people aren't aware of it at all.

Yeah we don't have a good system for this yet. I'm going to think on this a bit more, as I agree this should be added to CI as a test to ensure that all models with foreign keys or xyz_id properties to one of the 'root' objects are included in deletions.