Add custom 404 page to documentation (#21)

docs/.overrides/404.html

@@ -0,0 +1,50 @@
+{% extends "main.html" %}
+
+<!-- Content -->
+{% block content %}
+<style>
+    svg#clouds {
+        position: fixed;
+        width: 1920px;
+        z-index: -10;
+
+        left: -230px;
+        bottom: -4rem;
+
+        height: unset;
+        max-width: unset;
+    }
+</style>
+
+<script>
+    window.addEventListener("DOMContentLoaded", () => {
+        function get_path() {
+            const urlParams = new URLSearchParams(window.location.search);
+            const pathValue = urlParams.get('path');
+            return pathValue ? decodeURIComponent(pathValue) : "/unknown-path"
+        }
+
+        document.getElementById("path").textContent = get_path();
+    });
+</script>
+
+<div align="center">
+    <h1>404 - Page Not Found</h1>
+
+    <p style="opacity: .75;">
+        We're sorry, but we couldn't find a page at the path <strong><span id="path"></span></strong>.
+    </p>
+
+    <p style="opacity: .75;">
+        Try searching or go to <a href="{{ base_url }}">Pyventus' homepage</a>.
+    </p>
+</div>
+
+<svg id="clouds" xmlns="http://www.w3.org/2000/svg" width="2611.084" height="485.677" viewBox="0 0 2611.084 485.677">
+    <title>Gray Clouds Background</title>
+    <path id="Path_39" data-name="Path 39"
+          d="M2379.709,863.793c10-93-77-171-168-149-52-114-225-105-264,15-75,3-140,59-152,133-30,2.83-66.725,9.829-93.5,26.25-26.771-16.421-63.5-23.42-93.5-26.25-12-74-77-130-152-133-39-120-212-129-264-15-54.084-13.075-106.753,9.173-138.488,48.9-31.734-39.726-84.4-61.974-138.487-48.9-52-114-225-105-264,15a162.027,162.027,0,0,0-103.147,43.044c-30.633-45.365-87.1-72.091-145.206-58.044-52-114-225-105-264,15-75,3-140,59-152,133-53,5-127,23-130,83-2,42,35,72,70,86,49,20,106,18,157,5a165.625,165.625,0,0,0,120,0c47,94,178,113,251,33,61.112,8.015,113.854-5.72,150.492-29.764a165.62,165.62,0,0,0,110.861-3.236c47,94,178,113,251,33,31.385,4.116,60.563,2.495,86.487-3.311,25.924,5.806,55.1,7.427,86.488,3.311,73,80,204,61,251-33a165.625,165.625,0,0,0,120,0c51,13,108,15,157-5a147.188,147.188,0,0,0,33.5-18.694,147.217,147.217,0,0,0,33.5,18.694c49,20,106,18,157,5a165.625,165.625,0,0,0,120,0c47,94,178,113,251,33C2446.709,1093.793,2554.709,922.793,2379.709,863.793Z"
+          transform="translate(142.69 -634.312)" fill="var(--md-default-fg-color--lightest)"/>
+</svg>
+
+{% endblock %}