Llink new doc (Related Issues) from indexes pages
See the general Documentation guidelines http://docs.gitlab.com/ce/development/doc_styleguide.html
What does this MR do?
Links https://docs.gitlab.com/ee/user/project/issues/related_issues.html from https://docs.gitlab.com/ee/user/project/issues/index.html and from https://docs.gitlab.com/ee/README.md
cc @JobV @axil we need to make sure new features docs are linked from index pages whenever it exists, otherwise, our new features will stay hidden in our docs :|