Add basic set of error pages

app/views/layouts/application.html.slim

@@ -0,0 +1,20 @@
+doctype html
+html data-bs-theme="#{cookies[:"modeTheme"] || "light"}" data-controller="set-theme"
+  head
+    title a11ydive
+    meta[name="viewport" content="width=device-width,initial-scale=1"]
+    = csrf_meta_tags
+    = csp_meta_tag
+    = stylesheet_link_tag "application", "data-turbo-track": "reload"
+    = javascript_include_tag "application", "data-turbo-track": "reload", type: "module"
+  body
+    = render partial: "layouts/navigation"
+    .container-fluid
+      .row
+        = render partial: "layouts/sidebar"
+        main.col.ps-md-2.pt-2
+          .container-fluid
+            #main-content[data-controller="rich-text-link-targets"]
+              = yield
+    footer.container-fluid.mt-auto.border-top
+      = Rails.configuration.build_version && "Version: #{Rails.configuration.build_version}"