Build offline documentation for the stable branch as well (#4896)

This must be done on the `master` branch's CI workflow as scheduled
CI workflows only run on the repository's default branch.

.github/workflows/build_offline_docs.yml

@@ -1,5 +1,7 @@
 name: Build documentation for offline usage
 on:
+  push: # FIXME
+  pull_request: # FIXME
   schedule:
     # Every week on Monday at midnight (UTC).
     # This keeps the generated HTML documentation fresh.
@@ -8,9 +10,16 @@ on:
 jobs:
   build:
     runs-on: ubuntu-20.04
+    strategy:
+      matrix:
+        branch:
+          - master
+          - stable
     steps:
       - name: Checkout
         uses: actions/checkout@v2
+        with:
+          ref: ${{ matrix.branch }}
 
       - name: Install dependencies
         run: |
@@ -24,7 +33,7 @@ jobs:
 
       - uses: actions/upload-artifact@v2
         with:
-          name: godot-docs-html
+          name: godot-docs-html-${{ matrix.branch }}
           path: _build/html
           # Keep the current build and the previous build (in case a scheduled build failed).
           # This makes it more likely to have at least one successful build available at all times.