Add browser extension to the UserNav menu

Created by: AlicjaSuska

Part of RFC 221: Improve discoverability/awareness of the browser extension

Description

• Add 'Browser extension' (with icon) to the UserNavItem as the last item on the list for better discoverability.
• It links to this Docs page

Design documentation

• View in Figma

Open questions

• Is the download icon a proper representation for the browser extension. TBD - discussion in Figma