Add RSS/Atom feed

[knatten]

This is so people can subscribe to a feed with new questions as they get published. Currently the only way to do that is by paying attention on X/Mastodon/Bluesky.

This will probably look similar to how we generate the json feed that the app uses, so look at these files for inspiration:

• https://github.com/knatten/cppquiz/blob/master/deployment/dreamhost/cronjobs/dump_published.sh
• https://github.com/knatten/cppquiz/blob/master/quiz/management/commands/dump_published_questions.py