add generated files lists/patterns to .gitattributes

[rmuir]

Description

generated files can be specified in .gitattributes as generated for a couple bonuses in github:

• doesn't mess up source code analysis (counts etc)
• changes are folded/hidden by default in code reviews

It isn't much, but it would be nice to do. Maybe other tools respect it too (now or in the future) and it makes our lives easier?

https://docs.github.com/en/repositories/working-with-files/managing-files/customizing-how-changed-files-appear-on-github