The walkthrough builds up a decentralized git repository setup, but git-annex can also be used with a centralized git repository.
We have separate tutorials depending on where the centralized git repository is hosted.
On GitHub -- However, GitHub does not currently let git-annex store the contents of large files there. So, things get a little more complicated when using it.
On GitLab -- This service is similar to GitHub, but supports git-annex.
On your own server -- use any unix system with ssh and git and git-annex installed. A VPS, a home server, etc.