docs: document merge queue deadlock workaround and merge order (open-telemetry#5233)

Assisted-by: Gemini

Author: aabmass

.github/workflows/ci.yml

@@ -43,6 +43,9 @@ jobs:
       - contrib
     runs-on: ubuntu-latest
     steps:
+      # Skip alls-green check for post-release version bumps in merge_group.
+      # This avoids a deadlock where core and contrib repos wait for each other's releases.
+      # See RELEASING.md for more details.
       - name: Decide whether the needed jobs succeeded or failed
         if: ${{ !(github.event_name == 'merge_group' && contains(github.event.merge_group.head_commit.message, 'Update version to')) }}
         uses: re-actors/alls-green@release/v1

.github/workflows/templates/ci.yml.j2

@@ -43,6 +43,9 @@ jobs:
       - contrib
     runs-on: ubuntu-latest
     steps:
+      # Skip alls-green check for post-release version bumps in merge_group.
+      # This avoids a deadlock where core and contrib repos wait for each other's releases.
+      # See RELEASING.md for more details.
       - name: Decide whether the needed jobs succeeded or failed
         if: ${% raw %}{{ !(github.event_name == 'merge_group' && contains(github.event.merge_group.head_commit.message, 'Update version to')) }}{% endraw %}
         uses: re-actors/alls-green@release/v1

RELEASING.md

@@ -14,6 +14,7 @@
     * Release builds now should pass.
   * Merge the release PR.
   * Merge the PR to main (this can be done separately from [making the release](#making-the-release))
+    * ⚠️ **Merge `core` first**: To avoid a merge queue deadlock with `contrib`, you must merge the `core` PR to `main` first. We skip `alls-green` in the core merge queue ([ci.yml](https://github.com/open-telemetry/opentelemetry-python/blob/main/.github/workflows/ci.yml)) for this PR, which unblocks `contrib` once merged.
 
 ## Preparing a new patch release