Add files as generated to file `.gitattributes`

[ckunki]

When initializing a project with cookiecutter using PTB project-template, most of the GitHub workflows are generated by PTB / cookiecutter.

So by default, PTB could add these files as "generated" to file .gitattributes.

• File slow-checks.yml should not be marked as "generated"
• Paths inside file .gitattributes could start with a slash / to avoid matching files with identical names in subdirectories
• Maybe the following files and folders should me marked as "generated", as well?
  • .pre-commit-config.yaml
  • folder doc/_static/ (although GitHub only hides text files, not svg, when marked as "generated")
  • SECURITY.md

[ArBridgeman]

I think this should be done after the workflow:check and a ci check is implemented. The risk is that users easily overlook several workflow changes because we made assumptions in the base project.