Add checks deep-dive section with dangling-records case study

Introduce a new #checks section presenting domain, zone and service
checkers in three columns linking to the documentation, plus a
dangling-records case study spotlight. Rework discover copy toward
peace-of-mind framing and adjust section backgrounds and CTA layout.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

layouts/partials/cta-join.html

@@ -1,24 +1,21 @@
 <!-- Call to Action Section -->
 <section id="cta">
     <div class="container">
-        <div class="cta-content">
+        <div class="cta-content" style="text-wrap: balance">
             <h2 class="display-5 fw-bold mb-4">
                 {{ i18n "cta-account-title" }}
             </h2>
-            <div class="row mb-4">
-                <p
-                    class="offset-sm-1 col-sm-10 offset-md-2 col-md-8 lead text-light"
-                >
-                    {{ i18n "cta-account-text" }}
-                </p>
+            <div class="d-flex justify-content-center mb-4">
+                <p class="lead text-light">{{ i18n "cta-account-text" }}</p>
             </div>
             <a
-                href="/beta/"
+                href="/{{ .Site.Language.Lang }}/beta/"
                 class="btn btn-lg btn-light px-4"
                 style="border-radius: 2em"
                 data-umami-event="cta-join"
-                >{{ i18n "cta-account-button" }}</a
             >
+                {{ i18n "cta-account-button" }}
+            </a>
         </div>
     </div>
 </section>