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/introduction/index.md

@@ -0,0 +1,8 @@
+---
+html: introduction.html
+parent: docs.html
+template: pagetype-category.html.jinja
+top_nav_grouping: Article Types
+---
+# Introduction
+This is a quick introduction to the principal features of the XRP Ledger (XRPL). Read this to get a high level understanding of the XRPL, then you can read more about areas of particular interest. The XRP Ledger is a blockchain that permanently records digital transactions of tokens between accounts. The sections below expand on the concepts introduced in that sentence.