beta testing github-app

Here are some observations from testing the github-app on a [test repo](https://github.com/lukpueh/test-gittuf-github-app) following [the getting started guide](https://github.com/gittuf/github-app/blob/602a09678b3fc5e2a48f73a938b8724ccf14020a/docs/getting-started.md).

* For some reason the [installation docs](https://github.com/gittuf/github-app/blob/602a09678b3fc5e2a48f73a938b8724ccf14020a/docs/getting-started.md#installing-the-app-on-your-repository-full-lite) made me think that there would be any gittuf specific configuration steps during the install, Maybe because of sentence "The UI will walk you through the standard installation process. As a part of this..." . So I was surprised, that the installation was finished after I chose an account to install, and briefly thought the installation had failed.
* Removing the quoted phrase might make the instruction clearer.
* After the installation I was redirected to the top of the getting started document I was coming from, and already two thirds through reading.
* The redirect should lead somewhere, where I should go next after the installation.
* I then created a [PR](https://github.com/lukpueh/test-gittuf-github-app/pull/1) and noticed the gittuf-app check "PR is not mergeable" (and in the details: "More approvals are necessary for the PR to be mergeable.")
* Where does this policy come from?
* Is it hardcoded into the app?
* I couldn't find docs, or settings for this.
* Does the app install a gittuf policy into the git metadata?
* How many approvals are actually needed?
* Also gittuf and GitHub give me mixed signals about the mergeability of my PR.
- gittuf: Verify gittuf policy / PR is not mergable
- GitHub: Merging can be performed automatically / Merge pull request
* Despite gittuf saying otherwise, merging seems to work
* How can I see the merge attestation now?
* When I fetch locally after merge, I do find an RSL Reference Entry (requires git/gittuf specific knowledge)
```
git fetch origin "refs/gittuf/*:refs/gittuf/*" && git cat-file -p $(cat .git/refs/gittuf/reference-state-log)
```
* Are there no attestations because I didn't approve the PR?
* Second round: [PR with approval](https://github.com/lukpueh/test-gittuf-github-app/pull/2)
* One approval does not seem to be enough. gittuf still says "More approvals are necessary for the PR to be mergeable."
* GitHub says otherwise
* Merging and fetching locally, I still don't see any attestations. But refs/gittuf/reference-state-log points to the second RSL Reference Entry now.
* Maybe much of this can be solved with better documentation / user feedback. :)