doc: release process: Update the timeline for the release announcement

Update the timeline so that the release timeline announcement goes out two
weeks prior to rc1. This way developers have more time to ensure that their
features will make it to the next release. Also add a reminder of the
release timeline one week prior to rc1.

Signed-off-by: Johan Hedberg <johan.hedberg@silabs.com>

Author: jhedberg

doc/project/release_process.rst

@@ -167,10 +167,14 @@ Release Milestones
      - Review target milestones
      - Finalize target milestones for features in flight.
      - Release Engineering
-   * - T-4W
+   * - T-5W
      - Release Announcement
      - Release owner announces feature freeze and timeline for release.
      - Release Manager
+   * - T-4W
+     - Release Timeline reminder
+     - Release owner sends a reminder of the feature freeze and timeline for release.
+     - Release Manager
    * - T-3W
      - Feature Freeze (RC1)
      - No new features after RC1, ONLY stabilization and cosmetic changes, bug and doc