devops: Simplify deployment

Earlier a "production" git branch was used to keep the generated HTML
(along with a LICENSE file). This is an anti-pattern.

Hugo can generate output to different folder anyhow.

With the new setup, we will delete the production branch and keep only
the source files in git. The output will be directly generated on the server
with hugo and served by nginx from a separate folder.

Instead of git push, the deployment is triggered by
`ssh -t "./update-camp-website"`
which runs a script on the server.

Ideally, a webhook should do that automatically on any commits to master.
But that requires a webhook listening and can be considered later.