WIP: Add GitLab Helm Chart to the list of products

Closes https://gitlab.com/gitlab-com/gitlab-docs/issues/228

Review app http://add-gitlab-chart.178.62.207.141.xip.io/ee/install/kubernetes/gitlab_chart/

Todos

1. Accessible from both CE and EE https://gitlab.com/gitlab-com/gitlab-docs/merge_requests/291#note_86342549
2. Include it in the versioning process https://gitlab.com/gitlab-com/gitlab-docs/issues/228#note_90191264
3. Refactor the chart docs altogether and fix some broken links
4. Redirect https://docs.gitlab.com/ee/install/kubernetes/gitlab_chart.html to https://docs.gitlab.com/ee/install/kubernetes/gitlab_chart/index.html
5. The Edit this page button needs to be fixed since what we do here is not supported in the logic of /lib/helpers/edit_on_gitlab.rb.

Blocking

• If we pull the charts inside ce/install/kubernetes/, this only happens in the docs site while it's built. The charts will not be available under /help.