Make "Get started with Builds" Help Page link work properly

`help_page_path('ci/quick_start', 'README')` causes a link to `https://gitlab.com/help/ci%2Fquick_start/README` which in turn breaks every relative link on the page. This remedies the problem.

That said, this is probably a horrible way of fixing this issue, *but* it does work. I can’t find much information on linking this deeply with Rails routes.

If anyone has suggestions as to a better way of handling this, I'll gladly take them. I tried a few different things, but none of them really worked.

Resolves #14872.

cc: @axil @rspeicher @elstamey

See merge request !3561