Documentation: Remove reference to RST online editor.
rst.ninjs.org is not available anymore, but there are alternatives
listed in this doc.
@@ -423,10 +423,6 @@ Helpful Tools
There are a number of tools, online and offline, which can be used to preview
documents are you edit them:
-- `rst.ninjs.org <http://rst.ninjs.org/>`__
-
- An online rST editor/previewer
-
- `ReText <https://github.com/retext-project/retext>`__
A simple but powerful editor for Markdown and reStructuredText. ReText is
