Stand up the org GitHub App for backport CI

[mjnitz02]

Background

docs/BACKPORT.md documents an optional GitHub App token for the backport
workflow. When BACKPORT_APP_ID and BACKPORT_APP_PRIVATE_KEY are present,
backport PRs are opened by the App and CI runs on them automatically.

Without those secrets the workflow falls back to GITHUB_TOKEN. That works, but
PRs opened with GITHUB_TOKEN do not trigger other workflows, so CI never runs
on the backport PR. The workflow makes the gap loud (it applies a do not merge
label and comments the exact commands to kick CI by hand), but every backport
still needs a human to push an empty commit before it can be merged.

This repo has not stood up the App, so it runs on the fallback path itself.

What needs to happen

An org admin needs to:

1. Create a GitHub App under the datarobot-oss org, per the step-by-step in
   docs/BACKPORT.md.
   Permissions needed are Contents: Read and write and
   Pull requests: Read and write.
2. Install it on the repos that use backports.
3. Add BACKPORT_APP_ID and BACKPORT_APP_PRIVATE_KEY as org-level Actions
   secrets.

No workflow change is required in this repo or in any consuming repo. The
example workflows already pass both secrets through as optional, so the switch
from fallback to App happens the moment the secrets exist.

Why it is filed rather than done

It needs org-admin access and a decision about which repos the App is installed
on, so it cannot be resolved inside this repo. Filed so the gap is tracked
somewhere actionable instead of as a TODO in published documentation.