[WIP] refine the website structure

[haoqing0110]

More and more fields and operations are added to OCM, like rollout strategy, placement strategy, addon template, manfestworkreplicaset, etc, they are all mixed in the existing "Concept" section. This increases the mental load for OCM beginners, and makes users confused about like, addon vs manifestwork, addon-framework vs addon template.

This PR proposed adding new sections, User guides and FAQs to the OCM website.

• Concept: focus on what it is and how it works, the relationship between the key concepts. For example, what is ManagedCluster, how to register it to the hub, and how the cert rotates.
• User guides: what users can do. For example, how to register or uninstall a ManagedCluster.
• FAQ: frequently asked questions. For example, addon vs manifestwork vs addon vs manifestworkreplicaset, which one should be chosen to use.

If this can be discussed and approved, subsequent PR will be submitted, split the concept and user guide and fulfil the missing content.

[openshift-ci]

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[openshift-merge-robot]

PR needs rebase.

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