Hide Customizable dashboard user flows

The current Customisable dashboard experience (Creating, editing, cloning) can be quite confusing for the user (see pain points describe #525078 for example ), as the feature is not fully complete or working. While we figure out how to move the framework forward we decided to temporarily disable/hide it from the user.

We should also update the user docs to remove references to customisable dashboards.

This should be gated by a wip feature flag, so that we can easily toggle it on/off while we work on improving it.