Improve handbook sections related to GitLab EE development

This merge request

• moves relevant sections to developer-onboarding page from projects page
• fixes links to GitLab EE project section in the handbook

data/projects.yml

@@ -19,15 +19,9 @@ gitlab-ee:
     This is _not_ an open source project, but we made the source code available for
     viewing and contributions.
-    GitLab Community Edition is merged daily to GitLab Enterprise Edition (look for
-    the `CE Upstream` merge requests) and as a developer it's your responsibility
-    to ensure that your merge requests apply cleanly to GitLab EE.
-
-    GitLab EE requires a license key to be used. To be able to run your
-    own instances for development you can use one of the shared licenses
-    (like https://license.gitlab.com/licenses/1449) or you can clone
-    https://dev.gitlab.org/gitlab/license-app and generate your own
-    licenses.
+    GitLab Community Edition is merged daily to GitLab Enterprise Edition.
+
+    GitLab EE requires a license key to be used.
 gitlab-shell:
   name: GitLab Shell