OpenTDF docs should support dark mode / system selector

[marythought]

Looks like they can by enabled dark mode in docusaurus.config.ts:

• Set disableSwitch: false - Shows the theme toggle in the navbar
• Set respectPrefersColorScheme: true - Respects user's system preference

Now the docs will:

• Display a light/dark mode toggle switch
• Automatically match the user's system theme preference
• Allow manual toggling between themes
• Remember the user's choice across sessions

This colorMode setting in themeConfig applies to the entire site. It's a global Docusaurus theme configuration that affects:

• All documentation pages
• The homepage
• Blog pages
• API documentation pages
• Any custom pages
• The toggle switch will appear in the navbar site-wide, and the theme preference will be consistent across all pages as users navigate through the documentation.