Add dark/light mode toggle for documentation site

[Pokled]

Context

The documentation site at nodyx.dev currently only has one theme. A dark/light toggle would improve readability for users who prefer light mode.

What needs to be done

1. Add a toggle button in the documentation layout
2. Switch between dark and light CSS themes
3. Persist the choice in localStorage

Skills needed

• Basic CSS / HTML / JS
• No backend knowledge required