Modernize project

[LKajan]

Overall modernization of the project. No changes to the actual template yet. I'll provide another PR for that.

Main changes:

• Mark template files with .j2 extension
  • This is done to make it clear which files are actual templates and to enable easier use of jinja syntax highlighting extensions.
• Change linting to use Ruff.
  • Ruff is more comprehensive and makes linting faster For other changes check commit messages.