Docs don't explain critical step for using github-ci

The following is in the `web.yml` generated for github-ci, but it is *not* in the [hosting documentation](https://opensource.axo.dev/oranda/book/hosting.html).

```yml
# Note that once the gh-pages branch exists, you must
# go into repo's settings > pages and set "deploy from branch: gh-pages".
# The other defaults work fine.
```

Given the goal of this tool, make creating a repo site simple, calling out this arcane to many step would likely help quite a few people use your tool.