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>

wp-notes.php

@@ -740,12 +740,6 @@ function wp_notes_page_callback() {
             <a href="<?php echo esc_url(admin_url('admin.php?page=wp-notes-about')); ?>">Read more on the About page →</a>
         </p>
 
-        <div class="wp-notes-header-banner">
-            <img src="<?php echo esc_url(WP_NOTES_URL); ?>assets/wp-notes-banner.jpg"
-                 alt="WP Notes Banner"
-                 class="wp-notes-banner-img">
-        </div>
-
         <!-- Note Creation Form with WordPress Admin Styling -->
         <div class="postbox">
             <div class="postbox-header">