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/references/http-websocket-apis/admin-api-methods/key-generation-methods/index.md

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+---
+html: key-generation-methods.html
+parent: admin-api-methods.html
+template: pagetype-category.html.jinja
+---
+# Key Generation Methods
+Use these methods to generate and manage keys.

content/references/http-websocket-apis/admin-api-methods/logging-and-data-management-methods/index.md

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+---
+html: logging-and-data-management-methods.html
+parent: admin-api-methods.html
+template: pagetype-category.html.jinja
+---
+# Logging and Data Management Methods
+Use these methods to manage log levels and other data, such as ledgers.

content/references/http-websocket-apis/admin-api-methods/peer-management-methods/index.md

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+---
+html: peer-management-methods.html
+parent: admin-api-methods.html
+template: pagetype-category.html.jinja
+---
+# Peer Management Methods
+Use these methods to manage your server's peer-to-peer connections.

content/references/http-websocket-apis/admin-api-methods/server-control-methods/index.md

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+---
+html: server-control-methods.html
+parent: admin-api-methods.html
+template: pagetype-category.html.jinja
+---
+# Server Control Methods
+Use these methods to manage the rippled server.

content/references/http-websocket-apis/admin-api-methods/signing-methods/index.md

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+---
+html: signing-methods.html
+parent: admin-api-methods.html
+template: pagetype-category.html.jinja
+---
+# Signing Methods
+Use these methods to work with transactions.

content/references/http-websocket-apis/admin-api-methods/status-and-debugging-methods/index.md

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+---
+html: status-and-debugging-methods.html
+parent: admin-api-methods.html
+template: pagetype-category.html.jinja
+---
+# Status and Debugging Methods
+Use these methods to check the status of the network and server.