Refactor the Toolkit website

[csantanapr]

We are moving from gatsbyjs to mkdocs.

We are going to use this as a base https://squidfunk.github.io/mkdocs-material/

Reasons:

• Left Navigation with more than 2 levels
• Have a Table of contents Nav per page
• Mkdocs has a lower bar to entry for contributors, for the most part, plain markdown

[binnes]

When to push to beta so as not to loose content. Latest draft available here