Add artifact upload to site build.

When the build was triggered by a push to the main branch upload the
generated output as a GitHub Actions artifact in order to facilitate
deployment.

.github/workflows/ci.yaml

@@ -1,3 +1,5 @@
+---
+name: CI
 on:
   push:
     branches: ["main"]
@@ -13,3 +15,11 @@ jobs:
       - name: nanoc compile
         run: |
           earthly --ci +build
+  upload:
+    runs-on: ubuntu-22.04
+    if: ${{ github.event_name == "push" }}
+    needs: compile
+    steps:
+      - uses: actions/upload-pages-artifact@v3
+        with:
+          path: output