Re-style Documentation to fit with UN Web Styles

[albertkun]

Need

The UN has a web style guide with recommendations for navbars and other content. We should probably try to commit to staying within these guidelines as much as possible:

• https://www.un.org/styleguide/

Next steps

• Create custom navbar react component
• Alter colors and font sizing to match
• Decide whether to keep dark mode or not. If we use dark mode, we can follow the UN TV which is mostly a dark scheme: https://media.un.org/en/webtv