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Updates existing docs to include emails endpoints, and adds new docs for
emails in general docs, as well as SDK docs.
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> [!IMPORTANT]
> Introduces server-side email sending API and updates documentation to
include comprehensive guides and SDK references for email functionality.
>
> - **Behavior**:
> - Introduces `sendEmail` API in `route.tsx` for sending emails with
HTML or templates.
> - Handles errors like missing user IDs and schema errors.
> - **Documentation**:
> - Adds `concepts/emails.mdx` detailing email types, sending methods,
and configuration.
> - Updates `docs-platform.yml` and `meta.json` to include email
documentation.
> - Adds `sdk/types/email.mdx` for `SendEmailOptions` type reference.
> - **UI/Style**:
> - Adds badge style for `sendEmailOptions` in `method-layout.tsx`.
>
> <sup>This description was created by </sup>[<img alt="Ellipsis"
src="https://img.shields.io/badge/Ellipsis-blue?color=175173">](https://www.ellipsis.dev?ref=stack-auth%2Fstack-auth&utm_source=github&utm_medium=referral)<sup>
for 2edeb57734. You can
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summary. It will automatically update as commits are pushed.</sup>
----
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<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit
* **New Features**
* Introduced server-side email sending API with templates, theming,
variables, and notification categories.
* **Documentation**
* Added comprehensive Emails concept guide and SDK references
(sendEmail, SendEmailOptions).
* Extended SDK index and platform navigation to include Email docs for
Next/React/JS.
* Added an “Emails” functional tag to API docs and route metadata.
* **Style**
* Added a distinct badge style for SendEmailOptions in the docs UI.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->
---------
Co-authored-by: Konsti Wohlwend <n2d4xc@gmail.com>
