Improve styling to match webkit.org, and not rewrite the world

This commit build's on Martin Donath's work with not trying to override all
the variables and using a lot of the built-ins.

I am only doing minor restyling of the text to closely match webkit.org's style.
The rest of the styles should remain untouched. Colors are handled by over-riding the
MkDocs Material colors with Webkit's color scheme.

Author: stwrt

docs/index.md

@@ -1,3 +1,8 @@
+---
+hide:
+  - navigation
+---
+
 # WebKit Overview
 
 WebKit is a cross-platform web browser engine. On iOS and macOS, it powers Safari, Mail, iBooks, and many other applications.