(doc) Dynamically generate sections

Rather than hard-coding them.  Then, as the number of sections increase,
no additional changes will be required here.

docs/input/appendix/index.cshtml

@@ -0,0 +1,12 @@
+Title: Appendix
+Order: 10
+---
+
+<h1>Sections</h1>
+
+<ul>
+@foreach (IDocument child in OutputPages.GetChildrenOf(Document))
+{
+  <li>@Html.DocumentLink(child)</li>
+}
+</ul>

docs/input/appendix/index.md

@@ -1,10 +0,0 @@
-Title: Appendix
-Order: 10
----
-
-# Sections
-
-* [Styles](xref:styles)
-* [Colors](xref:colors)
-* [Borders](xref:borders)
-* [Emojis](xref:emojis)