Add simple 404 page (#737)

Co-authored-by: Carter Anderson <mcanders1@gmail.com>

sass/pages/_404.scss

@@ -0,0 +1,15 @@
+.fourohfour {
+  text-align: center;
+
+  h1 {
+    font-size: 8rem;
+    margin: 0;
+  }
+  p {
+    margin: 0 0 15px 0;
+    font-size: 1.5rem;
+  }
+  img {
+    width: 220px;
+  }
+}

sass/site.scss

@@ -1,21 +1,21 @@
 @charset "utf-8";
 
 // Core
-@import 'functions';
-@import 'vars';
-@import 'mixins';
+@import "functions";
+@import "vars";
+@import "mixins";
 
 // Fonts
-@import 'fonts/firamono';
-@import 'fonts/firasans';
+@import "fonts/firamono";
+@import "fonts/firasans";
 
 // Elements
 // - HTML element tweaks: html, body, a, ul, li…
-@import 'elements/html';
-@import 'elements/body';
+@import "elements/html";
+@import "elements/body";
 
 // CSS utility classes
-@import 'utils';
+@import "utils";
 
 // Components
 // - Repeating visual patterns: buttons, cards, menus…
@@ -54,3 +54,4 @@
 @import "pages/features";
 @import "pages/content";
 @import "pages/migration_guide";
+@import "pages/404";

templates/404.html

@@ -0,0 +1,8 @@
+{% extends "layouts/base.html" %}
+{% block content %}
+<div class="fourohfour">
+    <h1>404</h1>
+    <p>Sorry, we couldn't find that page.</p>
+    <img src="assets/bevy_icon_dark.svg" alt="Bevy logo">
+</div>
+{% endblock content %}

templates/layouts/base.html

@@ -1,6 +1,8 @@
 {% import "macros/header.html" as header_macros %}
 {% import "macros/public_draft.html" as public_draft %}
 
+{% set current_path = current_path | default(value="/") %}
+
 {% if section and section.title %}
     {% if section.path is starting_with("/learn/book/") %}
         {% set page_title = "Bevy Book: " ~ section.title %}