Update nox session `workflows:check` for modified workflows

[ArBridgeman]

Summary

In #641, we added the option to modify workflow templates via a YML using the nox session workflow:generate. Now, we want to ensure that the modified workflow templates are ONLY modified via that YML.

To Do

• Update workflow: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.
• 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