docs: Update AlchemySigner docs

[kevinweaver]

Is this a request for new documentation or updates to existing documentation
Existing Documentation

If existing documentation, a clear summary of what needs to be changed
The AlchemySigner docs suggest using the AlchemySigner type, which is deprecated with a comment suggesting the use of AlchemyWebSigner. This type is entirely absent from docs, so it's possible this comment is a typo as well, perhaps AlchemySmartSigner or SmartAccountSigner are intended?

If new documentation, a clear summary of what needs to be added
Please update the docs if AlchemySigner is truly deprecated and differentiate between:

• AlchemySigner
• AlchemyWebSigner
• AlchemySmartSigner
• SmartAccountSigner
• LocalAccountSigner
  as different docs point to each of these.

Additional context
Additional documentation issue here.

[moldy530]

this should all be up to date in the v4 docs now