ci(workflows): fix cron deploys not running on stable branches

GitHub Actions schedule events only fire on the default branch (master),
so stable* branches were building on push but never deploying.

Extract cron into a dedicated schedule-builds.yml orchestrator that
dispatches sphinxbuild.yml on master and every stable* branch via
workflow_dispatch. sphinxbuild.yml already deploys on workflow_dispatch,
so no deploy-condition changes are needed.

Signed-off-by: skjnldsv <skjnldsv@protonmail.com>

Author: skjnldsv

.github/workflows/schedule-builds.yml

@@ -0,0 +1,26 @@
+name: "Schedule documentation builds"
+
+on:
+  schedule:
+    - cron: '0 2 * * *'  # 02:00 UTC daily — triggers sphinxbuild on master + all stable* branches
+  workflow_dispatch:
+
+permissions:
+  actions: write
+
+jobs:
+  dispatch:
+    name: Dispatch sphinxbuild on all branches
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Dispatch on master and stable branches
+        env:
+          GH_TOKEN: ${{ github.token }}
+        run: |
+          set -e
+          for branch in master $(gh api "repos/${{ github.repository }}/branches" --paginate \
+            --jq '.[].name | select(startswith("stable"))'); do
+            echo "Dispatching sphinxbuild.yml on ${branch}"
+            gh workflow run sphinxbuild.yml --repo "${{ github.repository }}" --ref "${branch}"
+          done

.github/workflows/sphinxbuild.yml

@@ -6,8 +6,6 @@ on:
     branches:
       - master
       - stable*
-  schedule:
-    - cron: '0 2 * * *'  # 02:00 UTC daily — full build + deploy
   workflow_dispatch:
 
 permissions: