Mention detailed guide to setting up redis for sir-lancebot

[shtlrs]

Recently, a contrib had issues setting up redis on sir-lancebot, and they started running into connection problems.

Up until today, I ran into the same issue, and it was the fact that redis_use_fakeredis wasn't set in my .env variable.

After some debugging, I realized that it was a host issue, as it needs to be set to redis for the two containers to be able to talk to each other over the docker network.

Looking at the docs now, it's only mentioned in the extended guide of bot and not in sir-lance's, but it should be the case.