AssetHub Migration: move documentation, tests and examples to reference AssetHub instead of relay chain

[jsdw]

When Polkadot relay chain functionality is due to migrate to AssetHub, we should:

1. Update examples and documentation around Subxt to make use of AssetHub rather than the relay chain (including light client chainspecs).
2. Make it simpler for users to instantiate an AssetHubConfig, or provide one out of the box if possible (will be useful in order to do 1). This is complicated by the fact that our current attempts to build an AssetHubConfig rely on generated code to provide the AssetId type (a fairly complex MultiLocation type.