Move documentation to standalone website

[rclement]

Documenting the configuration of this plugin in the README was nice at first, but the more options added the less it becomes readable. And when adding generic widgets and such features, it will not be maintainable.

So it might be time to start moving the documentation to a standalone static website. Here is the plan:

• Use mkdocs documentation system
• Use mkdocs-material theme
• Deploy to GitHub Pages on rclement.github.io/datasette-dashboards/