Format extra help page text like wiki

The instance specific help text was not rendered in any way compatible
with the look and feel of the rest of the page.  Just re-use the wiki
style like the rest of the page.

app/views/help/index.html.haml

 %div
 - if Gitlab::CurrentSettings.help_page_text.present?
+  .prepend-top-default.md
     = markdown_field(Gitlab::CurrentSettings.current_application_settings, :help_page_text)
   %hr
 

changelogs/unreleased/55948-help-text-formatting-wiki.yml

+---
+title: Format extra help page text like wiki
+merge_request: 26782
+author: Bastian Blank
+type: fixed