feat: add 404.sql example

Author: wucke13

examples/handle-404/api/404.sql

@@ -0,0 +1,9 @@
+SELECT 'debug' AS component,
+    'api/404.sql' AS serving_file,
+    sqlpage.path() AS request_path;
+
+SELECT 'button' AS component;
+SELECT
+    'Back home' AS title,
+    'home' AS icon,
+    '/' AS link;

examples/handle-404/api/index.sql

@@ -0,0 +1,9 @@
+SELECT 
+    'title' AS component,
+    'Welcome to the API' AS contents;
+
+SELECT 'button' AS component;
+SELECT
+    'Back home' AS title,
+    'home' AS icon,
+    '/' AS link;

examples/handle-404/index.sql

@@ -0,0 +1,42 @@
+SELECT 'list' AS component,
+    'Navigation' AS title;
+
+SELECT
+    column1 AS title, column2 AS link, column3 AS description_md
+FROM (VALUES
+    ('Link to arbitrary path', '/api/does/not/actually/exist', 'Covered by `api/404.sql`'),
+    ('Link to arbitrary file', '/api/nothing.png', 'Covered by `api/404.sql`'),
+    ('Link to non-existing .sql file', '/api/inexistent.sql', 'Covered by `api/404.sql`'),
+    ('Link to 404 handler', '/api/404.sql', 'Actually `api/404.sql`'),
+    ('Link to API landing page', '/api', 'Covered by `api/index.sql`'),
+    ('Link to arbitrary broken path', '/backend/does/not/actually/exist', 'Not covered by anything, will yield a 404 error')
+);
+
+SELECT 'text' AS component, 
+    '
+# Overview
+
+This demo shows how a `404.sql` file can serve as a fallback error handler. Whenever a `404 Not
+Found` error would be emitted, instead a dedicated `404.sql` is called (if it exists) to serve the
+request. This is usefull in two scenarios:
+
+1. Providing custom 404 error pages.
+2. To provide content under dynamic paths.
+
+The former use-case is primarily of cosmetic nature, it allows for more informative, customized
+failure modes, enabling better UX. The latter use-case opens the door especially for REST API
+design, where dynamic paths are often used to convey arguments, i.e. `/api/resource/5` where `5` is
+the id of a resource.
+
+
+# Fallback Handler Selection
+
+When a normal request to either a `.sql` or a static file fails with `404`, the `404` error is
+intercepted. The reuquest path''s target directory is scanned for a `404.sql`. If it exists, it is
+called. Otherwise, the parent directory is scanned for `404.sql`, which will be called if it exists.
+This search traverses up until it reaches the `web_root`. If even the webroot does not contain a
+`404.sql`, then the original `404` error is served as response to the HTTP client.
+
+The fallback handler is not recursive; i.e. if anything causes another `404` during the call to a
+`404.sql`, then the request fails (emitting a `404` response to the HTTP client).
+     ' AS contents_md;