CI: split document and publish steps, and skip publish in forks.

.github/workflows/main.yaml

@@ -71,6 +71,21 @@ jobs:
       - name: Build documentation
         run: |
           sphinx-build docs docs/_build
+      - name: Upload documentation archive
+        uses: actions/upload-artifact@v3
+        with:
+          name: docs
+          path: docs/_build
+  publish-docs:
+    needs: document
+    if: github.repository == 'amaranth-lang/amaranth'
+    runs-on: ubuntu-latest
+    steps:
+      - name: Download documentation archive
+        uses: actions/download-artifact@v3
+        with:
+          name: docs
+          path: docs/
       - name: Publish development documentation
         if: github.event_name == 'push' && github.event.ref == 'refs/heads/main'
         uses: JamesIves/github-pages-deploy-action@releases/v4
@@ -78,7 +93,7 @@ jobs:
           repository-name: amaranth-lang/amaranth-lang.github.io
           ssh-key: ${{ secrets.PAGES_DEPLOY_KEY }}
           branch: main
-          folder: docs/_build
+          folder: docs/
           target-folder: docs/amaranth/latest/
       - name: Extract release version
         if: github.event_name == 'push' && startsWith(github.event.ref, 'refs/tags/v')
@@ -90,5 +105,5 @@ jobs:
           repository-name: amaranth-lang/amaranth-lang.github.io
           ssh-key: ${{ secrets.PAGES_DEPLOY_KEY }}
           branch: main
-          folder: docs/_build
+          folder: docs/
           target-folder: docs/amaranth/v${{ env.VERSION }}/