Wrap the sidebar contribution footer with proper spacing

Co-authored-by: Codex <codex@openai.com>

Author: rcosta358

_includes/nav_footer_custom.html

@@ -1,4 +1,4 @@
-<p>
-  Help improve this documentation by contributing on 
+<footer class="site-footer">
+  Help improve this documentation by contributing on
   <a href="https://github.com/liquid-java/liquidjava-docs">GitHub</a>
-</p>
+</footer>

_sass/custom/custom.scss

@@ -28,6 +28,10 @@
     $sidebar-color;
 }
 
+.site-footer {
+  color: $body-text-color;
+}
+
 .main-content a {
   color: #41586f;
   text-decoration-thickness: 0.08em;