[MB-9474] Add Service Objects and Validator Pattern guides

[sandy-wright]

Hi friends! Sorry this took a hot minute GAH, but I wrapped up these docs. There are a number of miscellaneous updates in this PR (sorry about that, got distracted by some iffy stuff in other backend docs), but the main docs to check are:

• docs/dev/contributing/backend/service-objects.md
• docs/dev/contributing/backend/service-validation.md

Try reading it locally (yarn start) to go through them. And then do whatever you want with this afterwards!

[rogeruiz]

I've added the MB-9474 Chore ticket 🔒 to this title so we can better track this work. In the future, it seems like this sort of work may need to be tracked under the MB-3672 Epic ticket 🔒 instead.

cc @pmohal

[sandy-wright]

heads up that I'm not allowed to merge this in, so someone else (@rogeruiz?) will need to do that when y'all think it's ready ☺️