* [docs] Upgrade to Docusaurus 2

Due to formatting issues with Docusaurus 1 and code blocks, and upcoming
4.3 and 5.0 releases, this is an upgrade to have more control over
formatting and other functionality.

This adds behavior to have light/dark themes.

As a consequence of upgrading, columnar format on ul element in
generated generator docs had to be moved to css. This will not impact
users viewing the markdown on GitHub because it didn't display the
column format.

This upgrade should improve syntax highlighting and performance.

* Fix user page layout, choose theme with clear diff colors