update: lemmy -> 0.18.4, @dgerard added as an infrastructure admin

some quick awful.systems infrastructure updates:

• @dgerard@awful.systems is now an infrastructure admin!
• updated lemmy to 0.18.4
• broke lemmy and lemmy-ui into their own flakes, which the deployment repo will grab and build as needed
• added the sneer-archive flake to the deployment
• finally wrote some docs on how to deploy from the flake