Update links to GitLab documentation site
Some links to GitLab Documentation in this repo returns 404 error.
For example: https://doc.gitlab.com/omnibus/settings/configuration.html#disable-user-and-group-account-management
This MR fixes and updates all links to GitLab Documentation site to start with https://docs.gitlab.com/.
Also, makes some redirects to lead to target page directly, and fixes some page anchors.
