docs: add note to direct to correct doc for self-hosted configuration (#46122)

## I have read the
[CONTRIBUTING.md](https://github.com/supabase/supabase/blob/master/CONTRIBUTING.md)
file.

YES

## What kind of change does this PR introduce?

Docs update

## What is the current behavior?

Received feedback that guide for customizing email templates for local
development was not helpful. Referenced variable that caused friction is
a variable related to self-hosted configuration.

## What is the new behavior?

Admonition added to direct users to correct guide for self-hosted
configuration.




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## Summary by CodeRabbit

* **Documentation**
* Added a note in the local development guide directing users to
self-hosting documentation for information about custom email templates.

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apps/docs/content/guides/local-development/customizing-email-templates.mdx

@@ -6,6 +6,12 @@ subtitle: 'Customize local email templates via the config file.'
 
 You can customize the email templates for local development by [editing the `config.toml` file](/docs/guides/local-development/cli/config#auth-config).
 
+<Admonition type="note">
+
+For configuring a self-hosted Supabase instance, see [Custom Email Templates](/docs/guides/self-hosting/custom-email-templates)
+
+</Admonition>
+
 ## Configuring templates
 
 You should provide a relative URL to the `content_path` parameter, pointing to an HTML file which contains the template. For example: