Move/rename indexes and some other files

This makes no difference to how the site is built under Dactyl (other
than where the "Edit Page" links go) but changes the URLs that will
be used for the pages after the migration to Redocly.

In addition to renaming index pages to index.md, I updated the style
guide to reflect the updated conventions, and moved a couple files that
were not in the correct folders for their place in the nav hierarchy.

content/references/protocol/ledger-data/index.md

@@ -0,0 +1,19 @@
+---
+html: ledger-data-formats.html
+parent: protocol-reference.html
+blurb: Learn about individual entries that comprise the XRP Ledger's shared state data.
+---
+# Ledger Data Formats
+
+Each [ledger version](ledgers.html) in the XRP Ledger is made up of three parts:
+
+- **[Ledger Header](ledger-header.html)**: Data about this ledger version itself.
+- **[Transaction Set](transaction-formats.html)**: The transactions that were executed to create this ledger version.
+- **[State Data](ledger-entry-types.html)**: A list of ledger entries, representing all accounts, settings, and balances as of this ledger version. (This is also called the "account state".)
+
+## State Data
+
+{% include '_snippets/ledger-objects-intro.md' %}
+
+{% from '_snippets/macros/page-children.md' import page_children with context %}
+{{ page_children(pages|selectattr("html", "eq", "ledger-object-types.html")|first, 1, 1, True) }}