Manual and game tell me, that I have to camp/rest to regenerate my bloodpoints, but both don`t say a single freaking word about how I can do that. And because there`s no keys section in the settings, I can`t look that up there either.
Please tell me: How the fuck do I rest?

There is no key for that, you need to find campfire to click on and choose if you want to rest.

in the first village you have the NPC - innkeep. he has an option to rent a room for 10 gold. somehow my campfire near the village stopped to work (no popup to click on it) so I use the inn.