adding docs delete action

Author: gilesknap

.github/workflows/docs_clean.yml

@@ -0,0 +1,31 @@
+name: Docs Cleanup CI
+
+# delete branch documentation when a branch is deleted
+# also allow manually deleting a documentation version
+on:
+  delete:
+  workflow_dispatch:
+    inputs:
+      version:
+        description: "documentation version"
+        required: true
+        type: string
+
+jobs:
+  remove:
+    if: github.event.ref_type == 'branch' || github.event_name == 'workflow_dispatch'
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: checkout
+        uses: actions/checkout@v2
+        with:
+          branch: gh-pages
+
+      - name: removing documentation for branch ${{ github.event.ref }}
+        if: ${{ github.event_name != 'workflow_dispatch' }}
+        run: echo rm -r .github/pages/${{ github.event.ref }}
+
+      - name: manually removing documentation version ${{ github.event.inputs.version }}
+        if: ${{ github.event_name == 'workflow_dispatch' }}
+        run: echo rm -r .github/pages/${{ github.event.inputs.version }}