Include APIManager CRD reference in Product document

For now, 3scale document links to the upstream document about APIManager CRD in Additional resources section.
There are some problems with it:

• Difficult to manage the urls. For example, The link in 2.9 document points to 3scale-2.8-stable-prod
• Customers have to refer to upstream documents.
• Upstream documents aren't official product documents.
• When there's a mistake in that, difficult to manage the issue as a product document issue. e.g. "apicast redis" is in HighAvailabilitySpec description, but it isn't managed.

Since APIManager CRD reference is essential information, please include it in the Product document.