[CORE-340] Submit success email

[EthelHsiao]

Type of changes

• Docs

Purpose

• Add sendResponseEmail to the form API contract
• Document whether a form should send a confirmation email to respondents after successful submission

Additional Information

• FormRequest now accepts optional sendResponseEmail; if omitted, backend should default it to false.
• FormResponse now returns required sendResponseEmail so frontend can initialize the settings toggle from form data.
• This PR only updates the API docs/contract. Backend storage, submission-time email decision logic, and email template implementation should be handled in the backend repo.
• Regenerated OpenAPI and Redocly docs successfully.

[github-actions]

🧹 PR is closed. Preview docs removed.

🔗 https://NYCU-SDC.github.io/core-system-api/preview-143/