Improve "convert repo" docs

[martinRenou]

I find the https://jupyter-releaser.readthedocs.io/en/latest/how_to_guides/convert_repo_from_repo.html docs page difficult to understand.

Set up a [GitHub App](https://docs.github.com/en/apps/creating-github-apps/about-creating-github-apps/about-creating-github-apps#github-apps-that-act-on-their-own-behalf) on your organization (or personal account for a personal project).
- Disable the web hook
- Enable Repository permissions > Contents > Read and write
- Select “Only on this account”
- Click “Create GitHub App”
- Browse to the App Settings
- Select “Install App” and install on all repositories
- Under “General” click “Generate a private key”
- Store the APP_ID and the private key in a secure location (Jupyter Vault if using a Jupyter Org)

Missing information from here:

• where to find how to create a Github app: org settings -> developper settings -> Github Apps -> New Github App button
• What to provide as Homepage URL?

Create a “release” environment on your repository and add an APP_ID Environment Variable and APP_PRIVATE_KEY secret. The environment should be enabled for “Protected branches only”.

This is a bit unclear. Where to find the APP_ID? Should the private key include -----BEGIN RSA PRIVATE KEY----- and -----END RSA PRIVATE KEY----- (yes)? We need to clarify this

Configure Rulesets for the repository Set up branch protection (with default rules) on publication branches Remove global tag protection. Add a branch Ruleset for all branches Allow the GitHub App to bypass protections Set up Pull Request and Required Checks Add a tags Ruleset for all tags Allow the GitHub App to bypass protections

The steps are unclear. I was personally not able to go through this. It seems it's not required though. Should we remove that from the docs or add more information and screenshots?