F OpenNebula/one#7652: Added custom 404 response page (#640)

This PR introduces a custom 404 response page which retains OpenNebula branding and Docs layout, plus the nav bar and some link suggestions such that the user can more easily find the documentation they were looking for. It also gives a more professional appearance.

Author: mattrowe-opennebula

layouts/_default/404.html

@@ -0,0 +1,34 @@
+{{ define "main" }}
+<style>
+.td-404 [role=main] {
+    width: 100% !important; /* Full width (10/12 columns) */
+    max-width: none !important;
+    margin-right: 0 !important;
+}
+</style>
+<div class="row flex-xl-nowrap" style="width:100%">
+    <main class="col-12 col-md-9 col-xl-8 ps-md-5" role="main">
+        
+        <div class="td-content">
+            <h1>Page Not Found</h1>
+            <p>The documentation for OpenNebula is regularly updated. It's possible that the page you are looking for has moved.</p>
+            <p>You may find what you are looking for in one of the following sections:</p>
+
+            <ul>
+                <li>
+                    <a href="{{ .Site.BaseURL }}/">Documentation Home</a>
+                </li>
+                {{ $paths := slice "getting_started/" "product/" "software/installation_process/" "software/upgrade_process/" "software/release_information/" "solutions/"}}
+                {{ range $paths }}
+                    {{ with $.Site.GetPage . }}
+                        <li><a href="{{ .RelPermalink }}">{{ .Title }}</a></li>
+                    {{ else }}
+                        {{/* Validation check */}}
+                        {{ errorf "404 Page: Link path not found: %s" . }}
+                    {{ end }}
+                {{ end }}
+            </ul>
+        </div>
+    </main>
+</div>
+{{ end }}