docs: add index pages for how-to and reference subdirectories

Author: wevans997-dev

src/content/docs/resource-tagging/how-to/index.mdx

@@ -0,0 +1,12 @@
+---
+title: How-to guides
+sidebar:
+  order: 3
+head:
+  - tag: title
+    content: How-to guides
+---
+
+import { DirectoryListing } from "~/components"
+
+<DirectoryListing />

src/content/docs/resource-tagging/reference/index.mdx

@@ -0,0 +1,12 @@
+---
+title: Reference
+sidebar:
+  order: 4
+head:
+  - tag: title
+    content: Reference
+---
+
+import { DirectoryListing } from "~/components"
+
+<DirectoryListing />