An external link was recently added but was broken because
'https://gitlab.com/help/' was prepended to every link in the page.
Since no link in the main help readme begins with "help" and since doing
so wouldn't make sense, the substitution conditionaly prepending "help"
can be simplified and reused.
Signed-off-by: David Wagner <david@marvid.fr>
Rémy Coutable