Update nox session `workflows:check` for `modified` workflows

[ArBridgeman]

Summary

In #641, we added the option to modify modified workflow templates via a YML using the nox session workflows:update. Now, we want to ensure that the modified workflow templates are ONLY modified via that YML, so we will extend the work from #639 so that workflows:check to also checks *modified templates.

To Do

• Update workflows:check to check that modified workflow templates are the same as what is present in the project. If not, print out the difference and exit 1. If they're identical, then exit 0. Note that the functions used in Allow projects to modify modified workflow templates with a YML #641 to perform the modifications also need to be used here
• Update the PTB so that checks.yml and slow-checks.yml will work
• Update the user & developer guides
• Update the release guide to nudge users into updating their .gitattributes files accordingly