This tries to revamp/restructure the installation guide. It's renamed to
"Getting Started" as it now contains a few more things than just the
installation, especially the deployment considerations which we didn't
use to spell out as much ahead of time.
Installation is now a section with the guides on their own. I've removed
a bit of redundancy like the user creation. I also removed the rogue
reverse proxy section in the Docker guide and lifted that into the
reverse proxy section.
