Added instructions for systemd and fixed small typos in the installation docs

[damoklov]

Your checklist for this pull request

• [x] A pull request against the devel branch.
• [x] Start branch off KLara's devel.
• [x] Check the commit's or even all commits' message styles matches our requested structure.

Description

1. Changed database's naming (kl-klara -> klara), as users create klara database at the beginning.
2. Changed database to insert API keys into (projetcs -> agents).
3. Added instruction how to enable systemd services via systemctl.

💔 Thank you!