Improve swap routing guide

[rocconmarco]

This PR introduces improvements in three main areas of the guide:

1. Missing imports: The sample code was missing imports for StateLibrary and the PoolKey type, which are required for the code to compile correctly.

2. Universal Router interface: Replacing the direct contract instantiation with the IUniversalRouter interface solves the following Solidity error: Explicit type conversion not allowed from non-payable "address" to "contract UniversalRouter", which has a payable fallback function. (solidity 7398)
   This also brings consistency with how other external contracts (PoolManager, Permit2) are handled via interfaces.

3. Currency type unwrap: Unwrapping key.currency1 using the Currency type from v4-core fixes the following error: Explicit type conversion not allowed from "Currency" to "contract IERC20". (solidity 9640)

[vercel]

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