Documentation explaining how to set up GitHub App as a code host

Created by: pjlast

As mentioned in #38731 (closed) , we need thorough documentation explaining how to create a GitHub App, the required scopes, where in the site config to set it up. Kind of similar to https://docs.sourcegraph.com/admin/external_service/github . Sentry also has great docs explaining this process which we can learn from: https://docs.sentry.io/product/integrations/source-code-mgmt/github

#38731 (closed) should preferably point to these docs