Add "Validation using online schema" documentation (#12)

* Initial plan

* Add content for 'Validation using online schema' section in advanced-topics.md

Co-authored-by: DannyvdSluijs <618940+DannyvdSluijs@users.noreply.github.com>

* Apply review suggestions: add () to Validator instantiation and explicit false to json_decode calls

Co-authored-by: DannyvdSluijs <618940+DannyvdSluijs@users.noreply.github.com>
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---------

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Co-authored-by: DannyvdSluijs <618940+DannyvdSluijs@users.noreply.github.com>

Author: Copilot

_docs/advanced-topics.md

@@ -64,7 +64,61 @@ if ($validator->isValid()) {
 ```
 
 ## Validation using online schema
-This paragraph needs to be written, want to help out? Checkout GitHub repo!
+Validation of a JSON document can be done using a schema hosted online. The validator will automatically fetch the schema from the provided URL using its built-in `UriRetriever`.
+
+```php
+<?php
+
+$data = json_decode(file_get_contents('data.json'), false);
+
+// Validate against an online schema by passing the URL as a $ref
+$validator = new JsonSchema\Validator();
+$validator->validate($data, (object)['$ref' => 'https://example.com/your/schema.json']);
+
+if ($validator->isValid()) {
+    echo "The supplied JSON validates against the schema.\n";
+} else {
+    echo "JSON does not validate. Violations:\n";
+    foreach ($validator->getErrors() as $error) {
+        printf("[%s] %s\n", $error['property'], $error['message']);
+    }
+}
+```
+
+If you need more control over how the remote schema is retrieved (e.g. to cache it or pre-load it), you can use `UriRetriever` and `SchemaStorage` explicitly:
+
+```php
+<?php
+
+use JsonSchema\SchemaStorage;
+use JsonSchema\Validator;
+use JsonSchema\Constraints\Factory;
+use JsonSchema\Uri\UriRetriever;
+
+$schemaUrl = 'https://example.com/your/schema.json';
+
+// Fetch the remote schema
+$retriever = new UriRetriever();
+$schema = $retriever->retrieve($schemaUrl);
+
+// Register the fetched schema so that any $ref inside it resolves correctly
+$schemaStorage = new SchemaStorage($retriever);
+$schemaStorage->addSchema($schemaUrl, $schema);
+
+$validator = new Validator(new Factory($schemaStorage));
+
+$data = json_decode(file_get_contents('data.json'), false);
+$validator->validate($data, $schema);
+
+if ($validator->isValid()) {
+    echo "The supplied JSON validates against the schema.\n";
+} else {
+    echo "JSON does not validate. Violations:\n";
+    foreach ($validator->getErrors() as $error) {
+        printf("[%s] %s\n", $error['property'], $error['message']);
+    }
+}
+```
 
 ## Using custom error messages
 This paragraph needs to be written, want to help out? Checkout GitHub repo!