docs: respect system color scheme preference in Docusaurus (#16748)

Configure Docusaurus to automatically detect and respect the user's
system/browser color scheme preference (prefers-color-scheme).

This improves UX by matching the documentation theme to the user's
OS-level dark/light mode setting, while still allowing manual override
via the theme switcher.

Changes:
- Add colorMode configuration to docusaurus.config.js
- Enable respectPrefersColorScheme option

docs/my-website/docusaurus.config.js

@@ -231,6 +231,11 @@ const config = {
         ],
         copyright: `Copyright © ${new Date().getFullYear()} liteLLM`,
       },
+      colorMode: {
+        defaultMode: 'light',
+        disableSwitch: false,
+        respectPrefersColorScheme: true,
+      },
       prism: {
         theme: lightCodeTheme,
         darkTheme: darkCodeTheme,