[Moin-user] Link to HelpOnTemplates from NonexistentPage

[Gnarlodious]

I propose adding a link on the NonexistentPage page to the
HelpOnTemplates page. The text on that page that says "or use one of
the page templates" could point to the HelpOnTemplates page so users
could get an explanation of what template is best suited to their
immediate need.

-- Gnarlie