Order documentation navigation sections.

docs/how-to/index.md

@@ -1,5 +1,6 @@
 ---
 eleventyNavigation:
   key: How-to guides
+  order: 1
 permalink: false
 ---

docs/reference/index.md

@@ -1,5 +1,6 @@
 ---
 eleventyNavigation:
   key: Reference guides
+  order: 2
 permalink: false
 ---