Improve clarity of Helm Chart docs

We should:

• Reduce visual clutter of the index page
• Make it extremely clear that the chart nearly everyone should use is gitlab-omnibus
• Provide a summary overview of the charts at the top of the index page
• Link back to index on each individual chart
• Document the consequences of cloud-native charts, e.g. a migration with downtime
• Clean up language around supported cloud providers
• Clarify that these should be used for smaller deployments, HA & Geo not supported.

Closes #2748 (closed)