Docs: Improve our signed extension / ExtrinsicParams examples

[jsdw]

Currently, we have two examples related to these:

https://docs.rs/subxt/0.33.0/subxt/book/setup/config/index.html#implementing-and-adding-new-signed-extensions-to-the-config

and

https://docs.rs/subxt/0.33.0/subxt/book/setup/config/index.html#implementing-crateconfigextrinsicparams-from-scratch

The first example is OK I think, but the second example conflates two things; creating a custom ExtrinsicParams implementation, and using MultiLocation based AssetIds for the ChargeAssetTxPayment signed extension.

I think we should:

1. Remove the MultiLocation stuff from the second example.
2. Add a third example which is dedicated specifically to using the generated MultiLocation (which is something chains like asset hub require if you want to be able to pay tips when submitting extrinsics).

@bee344 we've beenb chatting about this in the Subxt Lounge; I thought it best to make a proper issue for it so that it's not lost! You're welcome to have a go at this (or we can discuss alternatives here), or else I'm sure we'll get around to addressing it in the new year :)