CI: add spine-plugin build step to docs workflow

The Documentation & Examples workflow builds each package explicitly
before building examples, but was missing the spine-plugin step,
causing Vite to fail resolving @melonjs/spine-plugin.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: obiot

.github/workflows/docs.yml

@@ -36,6 +36,8 @@ jobs:
         run: pnpm -F @melonjs/debug-plugin build
       - name: Build tiled-inflate-plugin
         run: pnpm -F @melonjs/tiled-inflate-plugin build
+      - name: Build spine-plugin
+        run: pnpm -F @melonjs/spine-plugin build
       - name: Build API docs
         run: pnpm doc
       - name: Build examples