Docs: Add documentation page for AppSync GraphQL single resolvers feature

[dreamorosi]

What were you searching in the docs?

In #3999 we added support for AppSync GraphQL single resolvers, but there is currently no documentation page for this new feature.

How can we improve?

We should add a dedicated documentation page with examples for using AppSync GraphQL single resolvers.

[powertools-for-aws-oss-automation]

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