Refactor the sectioning of the /manage page

- Use the <section> HTML elements
- Add some spacing between sections to improve readability

core/src/main/resources/jenkins/model/Jenkins/manage.jelly

@@ -47,7 +47,9 @@ THE SOFTWARE.
     </div>
 
     <j:forEach var="category" items="${it.categorizedManagementLinks.entrySet()}">
+      <section class="manage-page__category">
         <h2>${category.key.label}</h2>
+
         <div class="manage-page__row">
         <j:forEach var="m" items="${category.value}">
           <j:set var="icon" value="${m.iconClassName != null ? m.iconClassName : m.iconFileName}" />
@@ -92,6 +94,7 @@ THE SOFTWARE.
           </j:if>
         </j:forEach>
         </div>
+      </section>
     </j:forEach>
 
   </l:main-panel>

war/src/main/less/modules/manage-jenkins.less

+.manage-page__category {
+  &:not(:last-child) {
+    margin-bottom: 1.5rem;
+  }
+}
 .manage-page__row {
   display: block;
   display: flex;