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/@i18n/ja/references/http-websocket-apis/public-api-methods/payment-channel-methods/index.md

@@ -0,0 +1,7 @@
+---
+html: payment-channel-methods.html
+parent: public-api-methods.html
+template: pagetype-category.html.jinja
+---
+# Payment Channelメソッド
+Payment Channelは、2者間で繰り返される一方向の支払いや一時的な与信を可能にするためのツールです。これらのメソッドを使用して、Payment Channelを使用することができます。