Re-add GitHub Actions workflow setup

with support for manual triggering,
see #74

.github/workflows/build.yml

@@ -0,0 +1,33 @@
+name: build
+
+on:
+  push:
+    branches: [site]
+  workflow_dispatch: {}
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions/setup-ruby@v1
+        with:
+          ruby-version: 2.7
+      - name: Bundler cache
+        uses: actions/cache@v2
+        with:
+          path: vendor/bundle
+          key: ${{ runner.os }}-bundler-${{ hashFiles('Gemfile.lock') }}
+          restore-keys: |-
+            ${{ runner.os }}-bundler-
+            ${{ runner.os }}-
+      - name: Install dependencies
+        run: bash script/install --deploy
+      - name: Build
+        run: bash script/build --deploy
+        env:
+          ALGOLIA_API_KEY: ${{ secrets.ALGOLIA_API_KEY }}
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          JEKYLL_ENV: production
+          JEKYLL_GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          PAGES_REPO_NWO: ${{ env.GITHUB_REPOSITORY }}