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/tutorials/tasks/index.md

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+---
+html: tasks.html
+parent: tutorials.html
+top_nav_grouping: Article Types
+template: pagetype-category.html.jinja
+---
+# Tasks
+Common tasks on the XRP Ledger.

content/tutorials/tasks/manage-account-settings/index.md

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+---
+html: manage-account-settings.html
+parent: tasks.html
+template: pagetype-category.html.jinja
+---
+# Manage Account Settings
+Set up your XRP Ledger account to send and receive payments the way you want it to.

content/tutorials/tasks/use-specialized-payment-types/index.md

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+---
+html: use-specialized-payment-types.html
+parent: tasks.html
+template: pagetype-category.html.jinja
+---
+# Use Specialized Payment Types
+Use advanced features like Escrow and Payment Channels to build smart applications on the XRP Ledger.

content/tutorials/tasks/use-specialized-payment-types/use-escrows/index.md

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+---
+html: use-escrows.html
+parent: use-specialized-payment-types.html
+template: pagetype-category.html.jinja
+---
+# Use Escrows
+The XRP Ledger supports escrows that can be executed only after a certain time has passed or a cryptographic condition has been fulfilled. Escrows can only send XRP, not issued currencies.

content/tutorials/tasks/use-tokens/index.md

@@ -0,0 +1,7 @@
+---
+html: use-tokens.html
+parent: tasks.html
+template: pagetype-category.html.jinja
+---
+# Use Tokens
+Create and trade tokens (fungible or otherwise) in the XRP Ledger.