cleanup style for new doc landing, quickstarts, and header

[lyleschemmerling]

At the moment the docs landing, the quickstart, and the docs header are comprised of a lightly-edited export from webflow. For maintainability we should clean this up.

• [ ] clean up and organize website-doc-landing.scss in fusionauth-style to remove unused styles, consolidate targets, improve readability
• [ ] remove webflow-specific ids and classes from docs index.html, quickstarts index.html, and the docs-header
• [ ] audit and remove unused tags classes and assets
• [ ] remove dependency on fa-docs.js (Which is a webflow library) and port needed functions to prime-managed javascript