docs(dev): validate and rate-limit outgoing mail in email.rst - #15708

Summary

email.rst showed how to send mail but not how to avoid turning the instance into a spam relay.

• Document validating recipients with IMailer::validateMailAddress() before send().
• Point to #[UserRateLimit] / #[AnonRateLimit] or ILimiter (see security.rst programmatic rate limiting).
• Warn that anonymous / public users must not freely make the instance send mail to a typed-in address.

Closes #15683

Test plan

• Sphinx build of the developer manual succeeds for the email page
• New Validate recipients and rate-limit sends section mentions validateMailAddress
• Section references #[UserRateLimit] / #[AnonRateLimit] or ILimiter and links programmatic rate limiting
• Warning covers anonymous / public users and typed-in addresses

AI disclosure

This PR was drafted with the help of AI coding assistants (Cursor agents / LLM-based tools), including the description. I am responsible for it and happy to rework anything that doesn't fit.

All commits carry an Assisted-by: Cursor:grok-4.7 trailer.