Small refinements to GitLab developer docs navigation

Updated solution

A smaller refinement of !2843 (merged).

Former solution

Contributes to: #1225.

Now that we have a very basic structure for GitLab development documentation, we can say that the GitLab Pages developer docs are for "feature development", and place it in the navigation accordingly.

• Before: https://docs.gitlab.com/ee/development/pages/
• After: http://eread-gitlab-pages-dev-docs-are-feature-guide.35.193.151.162.nip.io/ee/development/pages/

CC @sselhorn @marcel.amirault @kassio @susantacker @fneill @rdickenson