Mark workflow templates as generated or modified

[ArBridgeman]

Summary

In #638 a class was created to track all workflow templates, to extend this, we want to mark a template as either generated or modified. In #425, an initial investigation was done that found that some of the workflow templates are not modified by any project. As such, these should be marked as generated.

To Do

• Mark workflow templates as generated or modified
  • See Make PTB Workflow Templates Configurable #425 for the initial investigation. It's ok for now, if a template is currently in modified but we would want to be only generated. Please make one or more issues to follow up on these cases.
  • For Have PTB use workflows:update to update its workflows #640 to be easier, it's imagined that checks.yml and slow-checks.yml will be modified.
• Update the nox session workflows:check so that any template marked with generated must be identical or an error is tossed. If the template does not exist in the repo, then no error should be tossed. Additionally, for now, templates marked with modified can deviate and will be ignored (do not update/compare them).
• Add the nox session workflows:check as a new step in the workflow template checks.yml.
• Update the user & developer guide to indicate which templates are generated or modified. Describe how generated templates may not be modified under any circumstances.