docs: add payload.auth to local api (#9632)

### What?
https://payloadcms.com/docs does not document the `payload.auth`
feature.

### Why?
With custom components, there is no explanation as to how you can get
the current user on the server side. It is also something useful to know
for many other scenarios. A Discord user even mentioned today, that they
spent quite some time, trying to figure out how to get the current user,
while on the server.

While I don't think it's the cleanest "place" to document it, I think
its what makes the most sense, given the current state of the
documentation. I tried to follow the existing format as close as
possible. The comments are longer, but I feel the information is
absolutely necessary to provide.

Confirmed that formatting works as expected and there are no errors
parsing the addition:

![image](https://github.com/user-attachments/assets/c8e203cd-3c9c-482c-850b-2a16f3958344)


Fixes #9631

docs/local-api/overview.mdx

@@ -304,6 +304,27 @@ const result = await payload.delete({
 If a collection has [`Authentication`](/docs/authentication/overview) enabled, additional Local API operations will be
 available:
 
+### Auth
+
+```js
+// If you're using nextjs, you'll have to import headers from next/headers, like so:
+// import { headers as nextHeaders } from 'next/headers'
+
+// you'll also have to await headers inside your function, or component, like so:
+// const headers = await nextHeaders()
+
+// If you're using payload outside of NextJS, you'll have to provide headers accordingly.
+
+// result will be formatted as follows:
+// {
+//    permissions: { ... }, // object containing current user's permissions
+//    user: { ... }, // currently logged in user's document
+//    responseHeaders: { ... } // returned headers from the response
+// }
+
+const result = await payload.auth({headers})
+```
+
 ### Login
 
 ```js