Docs redesign (#728)

* Adding a dark mode
* Adding reference for types to summary pages
* Adding API Reference
* Adding modifiers to methods/fields/etc
* Minimizing files input
* Caching a lot of the output pages
* Cache only for each execution
* Adding API references to existing docs

docs/input/widgets/tree.md

@@ -4,6 +4,8 @@ Description: "The **Tree** widget can be used to render hierarchical data."
 Highlights:
     - Custom colors and styles for guidelines.
     - Include any *Spectre.Console* widgets as child nodes.
+Reference: T:Spectre.Console.Tree
+
 ---
 
 The `Tree` widget can be used to render hierarchical data.