First class support for RDoc wiki

Description

We support creating wiki in RDoc format, but there a few issues which prevent the process from being as smooth as creating wiki in Markdown:

• In the wiki editor we say:

  To link to a (new) page, simply type [Link Title](page-slug)

This syntax does not work for RDoc (see gitlab-foss#18287 (moved)). A working syntax for RDoc is here[link:additional-page] (http://stackoverflow.com/questions/2230954/is-there-any-way-to-easily-link-a-file-in-rdoc)

• Wiki editor preview does not render RDoc (see gitlab-foss#1193 (moved))
• "Add bold text", "Add italic text", "Insert a quote", "Insert code", "Add a task list" and "Attach a file" do not work in RDoc

Proposal

• Display correct hint how to create a link when "RDoc" is selected in the dropdown
• Make sure that relative links work when wiki is displayed in "Project" tab - see gitlab-foss#23806 (closed)
• Do not display the rich editor when "RDoc" is selected (no Preview tab, no buttons for inserting elements, no Attach file) - first iteration

Links

See gitlab-foss#24041 (moved) - analogous issue for AsciiDoc