docs: add 404 page

The new 404 page provides a clear and concise message to users when they attempt to access a non-existent page, enhancing the user experience and guiding them to the home page or search functionality.

Author: phodal

docs/404.html

@@ -0,0 +1,12 @@
+---
+layout: default
+title: 404
+permalink: /404
+nav_exclude: true
+search_exclude: true
+---
+
+<h1>Page not found</h1>
+
+<p>The page you requested could not be found. Try using the navigation {% if site.search_enabled != false %}or search {%
+    endif %}to find what you're looking for or go to this <a href="{{ '/' | relative_url }}">site's home page</a>.</p>