Add the documentation for the default broker class feature

[Leo6Leo]

Proposed Changes

• This is the supplimentary documentation for the new feature implemented in https://github.com/knative/eventing/pull/7631/files

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[Leo6Leo]

/hold until the broker config PR mentioned in the description get merged.

[Cali0707]

/assign

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This Pull Request is stale because it has been open for 90 days with no activity. It will automatically close after 30 more days of inactivity. Reopen with /reopen. Mark as fresh by adding the comment /remove-lifecycle stale.