[landing] introduce new header elements to header component

Summary:
This diff is the second part to updating the Header component to match the redesigns. The foucs of this diff is to introduce and layout the new elements needed for the header redesign. The new elements include:

1. Beta Badge
2. Team and Investors page nav replacing the App page nav

In this diff I also updated any typography/colors to use our design system created by Ted. I did not update the Github/Twitter colors, because those are one off instances that probably won't ever need to change, but if we want to create variables for them, I can do that.

Depends on D7835

Test Plan:
Please watch the demo video to see how the new Header component looks and behaves

iPhone:

iPad:

Desktop:

Responsive:

Reviewers: atul, kamil

Reviewed By: atul

Subscribers: ted, ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D7839