Implement preview for RDoc wiki pages

Everyone can contribute. Help move this issue forward while earning points, leveling up and collecting rewards.

• Close this issue

Summary

Previewing RDoc markup is not supported in the wiki editor.

Improvements

Implement preview for RDoc markup, building on the work done in #228885 for AsciiDoc.

Involved components

• app/assets/javascripts/behaviors/preview_markdown.js
• app/controllers/concerns/preview_markdown.rb
• app/services/preview_markdown_service.rb
• ee/app/services/ee/preview_markdown_service.rb