ux: move banner from Create-Note page to About page (side-by-side intro)

The banner image was the first thing under the page title on the
Create-Note page — taking significant vertical space before the user
saw the textarea. The Create-Note page is for *doing*, not for
*reading-about*.

CHANGES
- Banner removed from wp_notes_page_callback() — Create-Note page
  now opens straight into the form.
- Banner added to the top of the About page in a new side-by-side
  row: image on the left capped at 320px wide (not full-bleed), and
  a short intro paragraph + "Go to WP Notes →" CTA on the right.
  Stacks vertically on narrow screens via flex-wrap.
- The side-by-side block lives in its own .wp-notes-about-intro
  container so it doesn't tangle with the rest of the About page's
  pre-existing nested toggles (those are still on the Tier 2 list).
- .wp-notes-header-banner / .wp-notes-banner-img CSS removed from
  wp-notes-styles.php — no longer referenced anywhere.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

inc/wp-notes-styles.php

@@ -11,18 +11,6 @@ if (!defined('ABSPATH')) {
 function wp_notes_admin_styles() {
     ?>
     <style>
-        /* Header banner image */
-        .wp-notes-header-banner {
-            margin: 16px 0 24px;
-        }
-        .wp-notes-banner-img {
-            display: block;
-            max-width: 1200px;
-            width: 100%;
-            height: auto;
-            border-radius: 4px;
-        }
-
         /* Footer "buy me a coffee" — lives at the BOTTOM of the page */
         .wp-notes-footer-support {
             text-align: center;