starting with the docs

- jekyllrb preset for gh-pages with AdminLTE 3 layout
- rebuild menu structure like AdminLTE v2.4 docs
- modified npm/Plugins.js to update docs assets with every production run

docs/404.html

@@ -0,0 +1,26 @@
+---
+layout: default
+title: Error 404
+---
+
+  <div class="content-wrapper">
+    <section class="content-header">
+      <div class="container-fluid">
+        <div class="row mb-2">
+        </div>
+      </div>
+    </section>
+
+    <section class="content">
+      <div class="error-page">
+        <h2 class="headline text-warning"> 404</h2>
+        <div class="error-content">
+          <h3><i class="fas fa-exclamation-triangle text-warning"></i> Oops! Page not found.</h3>
+          <p>
+            We could not find the page you were looking for.
+            Meanwhile, you may <a href="{{ site.baseurl }}{% link index.md %}">return to index</a>.
+          </p>
+        </div>
+      </div>
+    </section>
+  </div>