Upgrade to latest DocFx version

[pkbullock]

This is a major jump, because of the new versions of Material etc.

[pkbullock]

Reference to #200 - Updates/rewrite to the plugin needed

[pkbullock]

This is a work in progress on a project outside this repo. Check out https://pkbullock.github.io/pnp-docfx-theme/
at repo: https://github.com/pkbullock/pnp-docfx-theme

[pkbullock]

Why is this open for so long? Its a pain to upgrade, I have custom skins and plugins, at the moment, a lot of work for very little result. The generator just works atm.