feat: add 404 layout

docs/404.rst

@@ -0,0 +1,7 @@
+:orphan:
+:layout: 404
+
+404
+===
+
+The requested URL was not found on this site.

docs/customisation/layouts.rst

@@ -129,3 +129,6 @@ This template offers a special container for buttons:
 
         `Docs </install/>`_
         `GitHub <https://github.com/lepture/shibuya>`_
+
+404 layout
+~~~~~~~~~~

src/shibuya/__init__.py

@@ -2,6 +2,7 @@
 from pathlib import Path
 from sphinx.application import Sphinx
 from sphinx.builders.html import StandaloneHTMLBuilder
+from sphinx.builders.dirhtml import DirectoryHTMLBuilder
 from .context import (
     normalize_pageurl,
     normalize_localtoc,
@@ -59,6 +60,16 @@ def _initialize_builder(app: Sphinx):
     if isinstance(app.builder, StandaloneHTMLBuilder):
         app.builder.highlighter.formatter = WrapLineFormatter
 
+    if isinstance(app.builder, DirectoryHTMLBuilder):
+        _get_outfilename = app.builder.get_outfilename
+
+        def get_outfilename(pagename: str) -> str:
+            if pagename == '404':
+                return (Path(app.builder.outdir) / '404.html').resolve()
+            return _get_outfilename(pagename)
+
+        app.builder.get_outfilename = get_outfilename
+
 
 def setup(app: Sphinx):
     """Entry point for sphinx theming."""

src/shibuya/theme/shibuya/layout/404.html

@@ -0,0 +1,28 @@
+{%- extends "layout.html" -%}
+
+{% block extrahead %}
+<style>
+  .not-found {
+    padding-top: calc(30vh - var(--sy-s-offset-top));
+    text-align: center;
+  }
+  .not-found-back .i-icon {
+    transform: rotate(180deg);
+  }
+  .yue a.headerlink {
+    display: none;
+  }
+</style>
+{% endblock %}
+
+{% block body %}
+<div class="not-found">
+  {{ body }}
+  <div class="not-found-back">
+    <a href="{{ pathto(root_doc) }}">
+      <i class="i-icon chevron"></i>
+      <span>Back to home</span>
+    </a>
+  </div>
+</div>
+{% endblock %}