Multisite: Add missing "contribute.php" file to User Dashboard.

This changeset adds a `wp-admin/user/contribute.php` file to allow the "Get Involved" link to work on User Admin.

This changeset prevents an unintended 404 that occurs when a logged-in user clicks the "Get Involved" link (located in the menu-bar) from inside of the multisite-specific User Dashboard.

Related: r56220, r56309, #23348.

Props ignatiusjeroe, audrasjb, tobiasbg.

Fixes #61122.

git-svn-id: https://develop.svn.wordpress.org/trunk@58118 602fd350-edb4-49c9-b593-d223f7449a82

Author: JJJ

src/wp-admin/user/contribute.php

@@ -0,0 +1,13 @@
+<?php
+/**
+ * User Dashboard Contribute administration panel.
+ *
+ * @package WordPress
+ * @subpackage Administration
+ * @since 6.6.0
+ */
+
+/** Load WordPress Administration Bootstrap */
+require_once __DIR__ . '/admin.php';
+
+require ABSPATH . 'wp-admin/contribute.php';