Update GHA to install docsy CSS build dependencies

.github/workflows/main.yml

@@ -121,6 +121,12 @@ jobs:
         run: |
           npm i
           npm run get-proposals
+      - name: "⚙️ npm — docsy"
+        # Install dependencies according to https://github.com/google/docsy/#prerequisites
+        run: |
+          cd themes/docsy
+          npm i
+          npm i --save-dev autoprefixer postcss-cli postcss
       - name: "⚙️ hugo"
         run: hugo --baseURL "${{ needs.calculate-baseurl.outputs.baseURL }}" -d "spec"