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/api/index.cshtml

@@ -0,0 +1,11 @@
+Title: API Reference
+Order: 10000
+---
+<p>The Spectre.Console API reference is organized by namespace. Each namespace has one or more classes for working with Console applications.</p>
+
+<ul>
+    @foreach (var child in OutputPages.GetChildrenOf(Document).OrderBy(i => i.GetTitle()))
+    {
+        <li>@Html.DocumentLink(child)</li>
+    }
+</ul>