[Spring 2026] Update Email Service SDK docs

[llam36]

Context

• We have an Email Service API doc ready, but we need to have users use our SDK instead of the API directly
• So we need an Email SDK doc

Requirement

• Understand how to use Email Service here
• Add to new content to this new Email SDK site here
• Based the doc structure and writing style on this sample File Service site here
• Imagine you are a new user to Email Service, the SDK doc should walk users through with sufficient explanation and code samples so they go from knowing nothing to being able to configure Email Service and send emails
• If any steps can be done using both the SDK and Dashboard, please include instruction for both options
• Refer to the juno-sdk repo's email service for any code-related information

When you're done:

• Let me know on Slack