Update multicluster guide to reuse quickstart-setup script

[david-martin]

Depends on quickstart-setup changes in Kuadrant/kuadrant-operator#590 & mgc links changes in Kuadrant/multicluster-gateway-controller#797
Part of Kuadrant/kuadrant-operator#495

The intention is to:

• update the existing guide to allow for changes to how multi cluster dns is done (without ocm)
• have a unified script for setting up 1 or many clusters to avoid code duplication
• be able to use the already existing follow-on 'secure-protect-connect' guide in each & every additional cluster (updated in Add multi cluster support to quickstart-setup script kuadrant-operator#590)
• move all ocm & mgc related content into a new sub-section of the 'How to' sidenav item.

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[jasonmadigan]

👀

[jasonmadigan]

/lgtm
/hold

(feel free to unhold when you want to deploy - think you can do that without a re-ack)

[david-martin]

/unhold