docs: add extras module documentation and Roq sitemap/diagram plugins

[ehsavoie]

Add a new Extras section to the docs site with an overview landing page and dedicated subpages for HTTP Clients (Vert.x, Android), Storage & Persistence (JPA Task Store, Push Notification Config Store), Replicated Queue Manager, and OpenTelemetry. Add the Extras entry to the sidebar navigation menu. Enable the Roq sitemap and diagram plugins with a sitemap.xml template.

[gemini-code-assist]

Code Review

This pull request introduces comprehensive documentation for the A2A Java SDK 'Extras' modules, covering alternative HTTP clients, JPA-backed storage and persistence, replicated queue management with Kafka, and OpenTelemetry observability. The review feedback highlights two issues in the documentation: an incorrect Maven property escaping syntax in the BOM example, and a copy-paste typo in the SQL schema for push notification configurations where the payload column is mistakenly named 'task_data' instead of 'config_data'.

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[gemini-code-assist]

In Maven resource filtering, the standard way to escape a property so that it is not interpolated during the build (and is instead rendered literally as ${org.a2aproject.sdk.version}) is \${property}. The current escaping $\{...} puts the backslash inside the expression delimiter, which will not be correctly processed by Maven and may render the backslash literally in the final documentation.

Suggested change

	<version>$\{org.a2aproject.sdk.version}</version>
	<version>\${org.a2aproject.sdk.version}</version>

[gemini-code-assist]

In the a2a_push_notification_configs table schema, the column for storing the configuration payload is named task_data. This appears to be a copy-paste typo from the a2a_tasks table schema. Consider renaming this column to config_data or push_notification_config_data to better reflect its contents and avoid confusion.

Suggested change

	CREATE TABLE a2a_push_notification_configs (
	task_id VARCHAR(255) NOT NULL,
	config_id VARCHAR(255) NOT NULL,
	task_data TEXT NOT NULL,
	PRIMARY KEY (task_id, config_id)
	);
	CREATE TABLE a2a_push_notification_configs (
	task_id VARCHAR(255) NOT NULL,
	config_id VARCHAR(255) NOT NULL,
	config_data TEXT NOT NULL,
	PRIMARY KEY (task_id, config_id)
	);