Revises descriptive text and examples at the top of the linkifiers
tab in the organization settings overlay to be shorter, have only
one example and updates help center link text for capitalization.
Adds a link to the help center in the form/input area for adding
a new linkifier, which is consitent with the code playgrounds tab,
and means the link would be there if the form was converted to a
modal.
Also, adds the `rendered_markdown` class to the HTML code elements,
via an HTML span element so that the specific CSS rules for code
elements with that class will be applied to these examples.
