Add `# Storage layout` section to `InvoiceExt3` doc comment

[Kingsman-99]

Description

InvoiceExt3 in contracts/split/src/types.rs is stored separately from InvoiceCore/InvoiceExt/InvoiceExt2 to avoid disturbing existing XDR layouts (as noted in the existing comment). The doc comment does not explain which storage key is used or how to find the key in storage_keys.rs. Adding a # Storage layout section would help developers navigating the codebase.

Acceptance Criteria

• Add a /// # Storage layout section to InvoiceExt3 in contracts/split/src/types.rs
• The section names the storage key enum variant used (e.g. InvoiceKey::Ext3(invoice_id)) and explains why it is stored separately
• cargo doc builds without warnings
• cargo test passes

Context

• Target file: contracts/split/src/types.rs
• Reference contracts/split/src/storage_keys.rs for the key variant name