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/rule.md

@@ -5,6 +5,7 @@ Description: "The **Rule** class is used to render a horizontal rule (line) to t
 Highlights:
     - Custom colors for line and title.
     - Specify left, center or right aligned title.
+Reference: T:Spectre.Console.Rule
 
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