Implement new dropdown menus with icons

[ILeandersson]

Designs are in Figma file V1 under 'Navbar'. Re missing icons: I am adding them to the Figma design today.

Thanks for creating this issue.

As an internal note, this issue overlaps with #125

[ILeandersson]

Here are updated screenshots with new icons and copy

The updated footer is below. Let's just leave this issue open until the footer has been updated.

I am closing this since the new issue ocaml/ocaml.org#416 now tracks the comment above.