init emails docs (#848)

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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`.
> 
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----


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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 <[email protected]>

docs/scripts/generate-functional-api-docs.mjs

@@ -13,6 +13,7 @@ const FUNCTIONAL_TAGS = [
   'API Keys', 
   'CLI Authentication',
   'Contact Channels',
+  'Emails',
   'Oauth', // Note: OpenAPI uses "Oauth" not "OAuth"
   'OTP',
   'Password',