feat: initialize opencode project with Astro and Starlight

- Add package.json for project configuration and dependencies
- Create content configuration for Starlight documentation
- Add example guide and reference markdown files
- Create index page for documentation with navigation and next steps
- Set up TypeScript configuration for the project

web/opencode/astro.config.mjs

@@ -0,0 +1,26 @@
+// @ts-check
+import { defineConfig } from 'astro/config';
+import starlight from '@astrojs/starlight';
+
+// https://astro.build/config
+export default defineConfig({
+	integrations: [
+		starlight({
+			title: 'My Docs',
+			social: [{ icon: 'github', label: 'GitHub', href: 'https://github.com/withastro/starlight' }],
+			sidebar: [
+				{
+					label: 'Guides',
+					items: [
+						// Each item here is one entry in the navigation menu.
+						{ label: 'Example Guide', slug: 'guides/example' },
+					],
+				},
+				{
+					label: 'Reference',
+					autogenerate: { directory: 'reference' },
+				},
+			],
+		}),
+	],
+});