Add "Validation using online schema" documentation

[Copilot]

The advanced-topics.md page had an empty placeholder for the "Validation using online schema" section. This adds concrete content covering both usage patterns.

Changes

• Simple usage: Validate against a remote schema by passing an HTTPS URL as $ref — the built-in UriRetriever fetches it automatically:

$validator = new JsonSchema\Validator;
$validator->validate($data, (object)['$ref' => 'https://example.com/your/schema.json']);

• Explicit retrieval: Use UriRetriever + SchemaStorage directly for cases requiring pre-loading or correct resolution of $ref chains within the remote schema:

$retriever = new UriRetriever();
$schema = $retriever->retrieve($schemaUrl);

$schemaStorage = new SchemaStorage($retriever);
$schemaStorage->addSchema($schemaUrl, $schema);

$validator = new Validator(new Factory($schemaStorage));
$validator->validate($data, $schema);

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Add content on https://jsonrainbow.github.io/docs/docs/advanced-topics.html#validation-using-online-schema</issue_title>
<issue_description>How to validate using an online schema is left empty in the docs. Content should be added.
See https://jsonrainbow.github.io/docs/docs/advanced-topics.html#validation-using-online-schema</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Add content on https://jsonrainbow.github.io/docs/docs/advanced-topics.html#validation-using-online-schema #3

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[DannyvdSluijs]

Suggested change

	$data = json_decode(file_get_contents('data.json'));
	$data = json_decode(file_get_contents('data.json'), false);

[Copilot]

Fixed in 0b74a15.