Improve menu and navigation header for mobile devices

On small screen device, we have a fixed header (Training, Localization, Outreach, User Research, Relay Operations, Onion Services), a breadcrumb and also a menu (a white square) with internal pages.

We should improve this navigation having mobile users on mind.

Some ideas that we can discuss/implement, only for mobile devices:

• Change the fixed header size so it can fit on small screen devices.
• Remove breadcrumbs, because you can easily swipe back on mobile

Screenshot how it looks now: