Add "Sign up" button to the nav-bar

Created by: valerybugakov

Closes #13372.

Changes

• Adds Sign up button to the nav-bar.
• Changes Sign in button label to Log in. Changes button style to secondary.
• Moves Docs and About into the dropdown.

Note: kept nav-links order consistent between dropdown and desktop nav-bar. Added comment about that to Figma file.

Screenshots

Desktop:

Tablet: