Fix header to top of content pages

[lynwilhelm]

Problem
Users are missing the header content when they scroll down long pages, especially on smaller screens.

Acceptance criteria
Header always stays fixed when scrolling occurs (Simularium landing page and Quick Start page)

Notes
Consult Lyndsay with any questions. No design should be required for this implementation ticket as the header already has a nice box shadow applied and the bar will have the appearance of floating above the page.