Asset Hub role changes after AHM on Polkadot

Pages to be edited:

https://wiki.polkadot.com/learn/learn-system-chains/
https://wiki.polkadot.com/learn/learn-assets/
https://wiki.polkadot.com/general/glossary/#asset-hub

For more info see: https://docs.google.com/document/d/1XR3vL2p4QV0wC7FrlC8eN-q62BqNFTFElbj21wEmMGg/edit?tab=t.0#heading=h.vxykbd6ai7n7