docs: make aws documentation work on reboots
Created by: ggilmore
Fixes #72
This PR improves our AWS docs by doing the following:
- Updates our recommended AMI to Amazon Linux 2
- Modifies our cloud-init script to use
systemctl
to make sure that the docker service (and our Sourcegraph container) starts up when the EC2 instance is rebooted - Fixes the update instructions to make sure that the upgraded version of Sourcegraph also starts up when the instance is rebooted