feat(docs): enhance documentation site with internationalization and layout updates

- Updated the demo API to support locale-based content rendering.
- Refactored the main layout to accommodate language-specific metadata and routing.
- Introduced new components for blog and documentation pages, ensuring proper language handling.
- Added styles for improved UI consistency across different sections.
- Removed deprecated layout files and streamlined the structure for better maintainability.

apps/docs/lib/i18n.ts

@@ -0,0 +1,25 @@
+import { defineI18n } from 'fumadocs-core/i18n';
+
+export const i18n = defineI18n({
+  defaultLanguage: 'zh',
+  languages: ['zh', 'en'],
+  hideLocale: 'default-locale',
+});
+
+export type Language = (typeof i18n)['languages'][number];
+
+export function isLanguage(value: string): value is Language {
+  return i18n.languages.some((lang) => lang === value);
+}
+
+/** HTML `lang` attribute values. */
+export const htmlLang: Record<Language, string> = {
+  zh: 'zh-CN',
+  en: 'en',
+};
+
+/** Locale ids understood by the desktop app / demo. */
+export const demoLocale: Record<Language, string> = {
+  zh: 'zh-CN',
+  en: 'en-US',
+};