Add md files for auto index pages

Part of IA and Redocly migration work. Create simple .md files to
represent the automatic index pages that previously only existed in
the dactyl-config.yml file. (The code used to do this is also part
of the commit but is, hopefully, no longer needed.)

content/concepts/index.md

@@ -0,0 +1,8 @@
+---
+html: concepts.html
+parent: docs.html
+top_nav_grouping: Article Types
+template: pagetype-category.html.jinja
+---
+# Concepts
+Learn the "what" and "why" behind fundamental aspects of the XRP Ledger.

content/concepts/payment-types/index.md

@@ -0,0 +1,7 @@
+---
+html: payment-types.html
+parent: concepts.html
+template: pagetype-category.html.jinja
+---
+# Payment Types
+The XRP Ledger supports point-to-point XRP payments alongside other, more specialized payment types.