Refactor documentation

Cleanup and refactor the documentation. This is needed before the charts go live under the docs site.

• MR that adds the charts to the docs site https://gitlab.com/gitlab-com/gitlab-docs/merge_requests/364
• Epic with MVC actions gitlab-org&572 (closed)

Review apps

Before	After
http://charts-old.178.62.207.141.xip.io/charts/	http://add-charts-top-level.178.62.207.141.xip.io/charts/

Closes https://gitlab.com/charts/gitlab/issues/1112

Pages left as-is

• advanced - stuff that can be used at installation time and later in the process. Maybe move under /installation. Leave as-is for now.
• architecture - how the charts are architectured, for development purposes, but I can see it being useful for someone deciding to jump into the charts. Leave as-is for now.
• backup-restore - Leave as-is for now. Maybe create a management dir and move it there in the future.
• charts - possibly rename this to sub-charts? otherwise the ending URL will be https://docs.gitlab.com/charts/charts/. One other option is to make the root dir of the docs site /kubernetes. Leave as-is for this iteration.
• releases - changelog of releases. How often is this updated? Can we deprecate this and transfer some info into another relevant page? Leave as-is for now.
• troubleshooting - Leave as-is.

Pages that moved location

• cloud/ - Move to installation/
• gitlab-qa/ - Move under development
• helm - Move under installation/tools.md
• kube-monkey - Move under development
• minikube - Move under development
• preparation/ - Move under development/
• installation/version-mappings.md -> doc/index.md#gitlab-version-mappings