[docs] Add docs for routing engine

Author: andrewnicols

docs/apis/subsystems/routing/index.md

@@ -0,0 +1,73 @@
+---
+title: Routing
+---
+
+<Since issueNumber="MDL-81301" version="4.5" />
+
+Moodle includes a powerful routing system based upon the [Slim Framework](https://www.slimframework.com/), and [FastRoute](https://github.com/nikic/FastRoute).
+
+Routes are defined by creating classes within the `route` L2 namespace of a component, and associating methods with the `\core\router\route` attribute. Routes have a Route _group_ identified by the L3 namespace, for example `api`. Unknown route groups are ignored.
+
+The currently supported list of route groups are:
+
+| Route Group | Namespace | URI Prefix | Purpose |
+| --- | --- | --- | --- |
+| API  | `api` | `/api/rest/v2` | REST Web Services |
+
+Routes may optionally describe additional metadata including:
+
+- path parameters
+- optional path parameters
+- header parameters
+- HTTP method types (GET, POST, and so on)
+- Responses
+- Examples
+
+When invoked, any parameter to the route method will be resolved using a combination of [Dependency Injection](../../core/di/index.md) and resolution of path, query, and header parameters.
+
+## Using the `route` attribute
+
+When applied to a method, the `\core\router\route` attribute will create a route. Class-level attributes can also be defined and are used to define a route prefix, and some defaults, but cannot handle a route directly.
+
+The path will be combined with the URI prefix described by the route _group_ (for example `api` has a prefix of `/api/rest/v2`), and the component (for example `mod_example`) to create a fully-qualified path.
+
+Route groups are pre-defined by the Routing Subsystem and will provide a URI prefix, relevant [Middleware](https://www.php-fig.org/psr/psr-15/meta/), and some rules -- as an example the `api` route group has a route prefix of `/api/rest/v2`, and will apply a range of Route Middleware to configure CORS, perform Input and Output Sanitisation, normalise paths, and more.
+
+:::note
+
+Any unknown Route Group will be ignored.
+
+:::
+
+In the following example, the namespace of the class has:
+
+- A Level 2 namespace of `route`; and
+- A Level 3 namespace of `api`.
+
+This relates to the `api` route group giving it a path prefix of `/api/rest/v2`.
+
+```php title="A simple route"
+// mod/example/classes/route/api/example.php
+namespace mod_example\route\api;
+
+use core\router\schema\response\payload_response;
+use Psr\Http\Message\ResponseInterface;
+use Psr\Http\Message\ServerRequestInterface;
+
+class example {
+    #[\core\router\route(
+        // Resolves to https://example.com/moodle/api/rest/v2/mod_example/example
+        path: '/example',
+    )]
+    public function example_route(
+        ServerRequestInterface $request,
+        ResponseInterface $response,
+    ): ResponseInterface {
+        return new payload_response(
+            request: $request,
+            response: $response,
+            payload: [],
+        );
+    }
+}
+```