Let `HelpController#show` support rendering the doc under jh

In Upstream, we can use help_page_url('user/project/integrations/zentao') to fetch a self-managed doc link,

and can view the doc by http://127.0.0.1:3000/help/user/project/integrations/zentao

(https://docs.gitlab.com/ee/user/project/integrations/zentao.html on SaaS).

I'd like to extend the HelpController#show to support the rendering the doc under jh.

Proposal

• If jh/user/project/integrations/zentao exists, use help_page_url('user/project/integrations/zentao') to fetch the link, and render JH doc by http://127.0.0.1:3000/help/user/project/integrations/zentao .

• If jh/user/project/integrations/zentao not exists and user/project/integrations/zentao exists, use help_page_url('user/project/integrations/zentao') to fetch the link, and render CE/EE doc by http://127.0.0.1:3000/help/user/project/integrations/zentao .