docs: add BMad Builder announcement banner to docs site (#2209)

Author: bmadcode

website/astro.config.mjs

@@ -46,6 +46,10 @@ export default defineConfig({
       title: 'BMAD Method',
       tagline: 'AI-driven agile development with specialized agents and workflows that scale from bug fixes to enterprise platforms.',
 
+      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>.',
+      },
+
       // i18n: locale config from shared module (website/src/lib/locales.mjs)
       defaultLocale: 'root',
       locales,