Add forgejo workflow

.forgejo/workflows/build.yaml

@@ -0,0 +1,16 @@
+on: 
+  push:
+    branches:
+      - main
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: https://git.madhouse-project.org/actions/nix/install@main
+      - name: Run Nix build
+        run: nix build --extra-experimental-features nix-command --extra-experimental-features flakes
+      - uses: actions/upload-artifact@v3
+        with: 
+          name: presentation.pdf
+          path: result