Update guide_pretix.rst to include a section on redis

[karlquint]

When checking the system health via the proposed GET request (see here: https://docs.pretix.eu/self-hosting/maintenance/#uptime-monitoring), it returned a message that "Cache was not available". I checked the pretix docs and adapted the section on redis in the .pretix.cfg

[nichtmax]

There is no redis running on fresh asteroids, users need to set it up first, this will lead to confusion.

[karlquint]

@nichtmax the instructions for pretix in the lab guide for pretix already state that Redis needs to be installed according to the Redis guide.