Re-levelization: move non-docs content, rename content→docs

For better URLs, the content folder has been renamed 'docs' and all
other files have been moved up a level. Also, non-docs images have been
moved to the static folder at the top level where they belong.

Many relative paths had to be fixed to make this work.

@i18n/ja/docs/tutorials/tasks/use-tokens/index.md

@@ -0,0 +1,12 @@
+---
+html: use-tokens.html
+parent: tasks.html
+metadata:
+  indexPage: true
+---
+# トークンの利用
+
+XRP Ledgerでトークン(代替型やそれ以外)を発行し、取引しましょう
+
+
+{% child-pages /%}