Add Asset-Hub-Kusama, and Asset-Hub-Polkadot to Metadata tab

[TarikGul]

With the Asset-Hub migration under way it would nice to ensure we have docs for the kusama, and polkadot asset hub chains under the metadata tab. This is important because as the pallets are migrated from the relay chains sooner or later those methods will be gone, but will also fade from the relay chain metadata docs.