## Description
Adds Swagger API documentation for branded email layouts, including the
account-level layout endpoint, Email inbox update field, and generated
API schemas. This stacked PR keeps the implementation review in #14936
focused on the product change.
Related:
[CW-7514](https://linear.app/chatwoot/issue/CW-7514/branded-html-email-templates-per-inboxbrand)
Depends on #14936
## Type of change
- [x] This change requires a documentation update
## How to test
1. Run `bundle exec rake swagger:build`.
2. Start Chatwoot locally and open `http://localhost:3000/swagger`.
3. Verify the branded email layout account endpoint and Email inbox
`branded_email_layout` field appear in the API reference.
## Checklist
- [x] I have performed a self-review of my changes
- [x] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
