docs: add BMad Builder announcement banner to docs site (#15)

Author: bmadcode

website/astro.config.mjs

@@ -41,6 +41,10 @@ export default defineConfig({
       title: 'BMad Game Dev Studio',
       tagline: 'Game development (Unity, Unreal, Godot) module for the BMad Method.',
 
+      banner: {
+        content: 'Build your own BMad modules and share them with the community! <a href="https://bmad-builder-docs.bmad-method.org/tutorials/build-your-first-module/">Get started</a> or <a href="https://bmad-builder-docs.bmad-method.org/how-to/distribute-your-module/">submit to the marketplace</a>.',
+      },
+
       favicon: '/favicon.ico',
 
       // Social links