Add workflow to build docs without publishing them (dotnet#6691)

.github/workflows/docs.yml

@@ -0,0 +1,20 @@
+name: Build and publish site
+on:
+  workflow_dispatch:
+  push:
+    branches:
+      - docs
+
+jobs:
+  build:
+    runs-on: windows-latest
+    steps:
+    - uses: actions/checkout@v2
+    - name: Setup .NET Core 3.1
+      uses: actions/setup-dotnet@v1
+      with:
+        dotnet-version: '3.1.x'
+    - name: Setup .NET
+      uses: actions/setup-dotnet@v1
+    - name: Build
+      run: dotnet build src