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/use-cases/payments/index.md

@@ -0,0 +1,8 @@
+---
+html: payments-uc.html
+parent: use-cases.html
+top_nav_grouping: Article Types
+template: pagetype-category.html.jinja
+---
+# Payments
+Practical examples for using different types of payment on the XRP Ledger.