Docs: fix duplicate sections (CI docs)

See the general Documentation guidelines http://docs.gitlab.com/ce/development/doc_styleguide.html

What does this MR do?

• Delete duplicate sections on EE-docs (prob originated by merge conflicts on the CE > EE upstream):
  • https://docs.gitlab.com/ee/ci/README.html#gitlab-ci-cd-for-docker
  • https://docs.gitlab.com/ee/administration/index.html#customizing-gitlab-39-s-appearance
• Link CI doc to admin doc (backport to CE)
• Move sections for better reading flow (backport to CE)

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/41138 and https://gitlab.com/gitlab-org/gitlab-ce/issues/41872