Make the docs URL configurable in admin area

Description

This is a sub-issue of https://gitlab.com/gitlab-org/gitlab-ce/issues/18739.

Removing support of inline docs will render useless many internal links that we use in various UI areas. We need a solution for this.

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From https://gitlab.com/gitlab-org/gitlab-ce/issues/18739#note_29211966

Some environments do not allow user workstations to access the internet. A solution to this would be to allow GitLab server admins to modify the URL that /help leads to and allow admins to host the gitlab-com/gitlab-docs static site on their own servers.

Proposal

One possible solution would be to make the docs URL configurable in gitlab.yml or in GitLab admin area. That way it's only a matter of setting a default with each release and sysadmins can always take our docs portal build it on-premise and change the URL to point to theirs. The default URL would be pointing to docs.gitlab.com.

• That implies that the helper method help_page_path should be modified and change from <gitlab_url>/help/ to <docs_url>/ce/ or <docs_url>/ee/ for CE and EE respectively.

  Example:

  • = link_to 'Learn more', help_page_path('user/profile/preferences', anchor: 'syntax-highlighting-theme'), target: '_blank' would render to <gitlab_url>/help/user/profile/preferences.md#syntax-highlighting-theme. That would need to change to <docs_url>/ce/user/profile/preferences.html#syntax-highlighting-theme. Notice the html extension.

• All URLs should open in a new link (target: '_blank').

• Handling multiple versions should be taken into account too. What happens if a link on 9.2 points to the latest docs that run 9.7?