Add Enso zap/router integration docs and partner template guidance

[rossgalloway]

Summary

Add Enso zap/router integration guidance to the docs, and consider including a reusable partner integration template for external distribution/frontends.

Motivation

Distribution partners should have a clearer path to enabling zap deposits into Yearn vaults on their frontends.

Proposed changes

• document Enso zap and router integration basics in the docs
• add or link a partner-facing integration template / dev-doc example
• clarify the minimum requirements for enabling zap deposits into Yearn vaults

Acceptance criteria

• docs explain the Enso zap/router integration path clearly
• partner-facing implementation guidance exists in a reusable form
• a frontend/distribution partner can understand how to support zap deposits into Yearn vaults

Tracker

• To-do ID: T-008