feat(4): Replace workflow with three

.forgejo/workflows/ci.yml

@@ -0,0 +1,19 @@
+name: CI
+
+on:
+  push:
+  pull_request:
+
+jobs:
+  build:
+    runs-on: self-hosted
+    steps:
+      - uses: actions/checkout@v4
+      - name: Build
+        run: nix shell nixpkgs#mdbook --command ./deploy.sh build
+      - name: Upload artifact
+        uses: actions/upload-artifact@v3
+        with:
+          name: site-${{ github.sha }}
+          path: blog/
+