Expose /sitemap.xml for Bing + IndexNow

Astro's sitemap integration emits /sitemap-index.xml + /sitemap-0.xml. Bing
Webmaster Tools and IndexNow probe /sitemap.xml literally, so a request
for the canonical name was 404'ing. Two changes:

- astro.config.mjs: add a 301 redirect /sitemap.xml → /sitemap-index.xml
  (alongside the existing / → /en/ redirect)
- public/robots.txt: list both Sitemap URLs so any crawler that reads
  robots.txt finds an entry it can use directly

After production redeploys (`docker compose up -d --build`), submit
https://nibiru-framework.com/sitemap.xml in Bing Webmaster Tools — it'll
follow the 301 and ingest the index.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/astro.config.mjs

@@ -15,6 +15,10 @@ export default defineConfig({
 	devToolbar: { enabled: false },
 	redirects: {
 		'/': '/en/',
+		// Bing/Yandex/IndexNow probe /sitemap.xml directly. Astro's sitemap
+		// integration emits sitemap-index.xml + sitemap-0.xml; add a 301 so
+		// either path resolves. Both are also referenced in robots.txt.
+		'/sitemap.xml': '/sitemap-index.xml',
 	},
 	integrations: [
 		AstroPWA({