## Description
The Slack notification workflow for Desktop release notes PRs fails when
the PR is opened from a fork because GitHub doesn't expose repository
secrets to `pull_request` events from forks.
This PR splits the workflow into two:
1. **Release Notes PR Trigger** — runs on `pull_request`, saves PR
details (URL, title, author) as an artifact. No secrets needed.
2. **Notify Slack on Desktop Release Notes PR** — runs on `workflow_run`
after the trigger completes, downloads the artifact, and sends the
Slack notification. Has access to secrets since `workflow_run` runs in
the context of the base repository.
## Related issues or tickets
Fixes the `SlackError: Missing input! A token must be provided` error
in the notify-release-notes-pr workflow.
## Reviews
- [x] Technical review
Signed-off-by: Lorena Rangel <lorena.rangel@docker.com>
