Add the charts repo in the top level

Another implementation of https://gitlab.com/gitlab-com/gitlab-docs/merge_requests/291, but this time the charts are under /charts.

Review app: http://add-charts-top-level.178.62.207.141.xip.io/charts/

• closes https://gitlab.com/gitlab-com/gitlab-docs/issues/228
• part of epic &572 (closed)

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• Requires https://gitlab.com/charts/gitlab/merge_requests/588 first
• Charts versioning https://gitlab.com/gitlab-com/gitlab-docs/issues/329