ci: allow workflow dispatches

.github/workflows/documentation.yml

@@ -7,6 +7,11 @@ on:
       - '!docs'
     tags:
       - '**'
+  workflow_dispatch:
+    inputs:
+      ref:
+        description: 'The branch, tag or SHA to checkout'
+        required: true
 jobs:
   build:
     name: Build documentation
@@ -19,6 +24,8 @@ jobs:
     steps:
       - name: Checkout repository
         uses: actions/checkout@v2
+        with:
+          ref: github.event.inputs.ref || ''
 
       - name: Install node.js v16
         uses: actions/setup-node@v2