Reorganized sections for semantics and outline

+ moved the masthead outside of main and article
+ fixed use of <h1> so that it coincided with <title> for SEO reasons.
+ reorganized sections and normalized associated <h?> tags.
+ ran this though https://gsnedders.html5.org/outliner/ to make sure it
produced a sensible outline.
Cleaned up some text and formatting

+ Fixed typos
+ Cleaned up some formatting.
+ Removed redundant section on Typescript/Dart visibility.