chore: Update codeowners to remove MetaMask/core-platform from docs folder (#167)

This changes `CODEOWNERS` so `MetaMask/core-platform` owns everything,
except the `docs` folder.

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Low Risk**
> Low risk because it only changes review ownership rules and does not
affect runtime behavior or shipped code.
> 
> **Overview**
> Adjusts `.github/CODEOWNERS` to keep `@MetaMask/core-platform` as the
default owner for `*`, but explicitly excludes `/docs` so it has no
specific code owner.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
9a50ce4. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: Mrtenz

.github/CODEOWNERS

@@ -2,3 +2,6 @@
 # Each line is a file pattern followed by one or more owners.
 
 * @MetaMask/core-platform
+
+# Docs folder is not owned by any specific team.
+/docs