docs: BlueOS extension extras manifest is undocumented

A BlueOS extension can ship widgets, actions and joystick mapping suggestions to Cockpit by putting a path in `metadata.extras.cockpit` and serving an `ExtrasJson` file there. That contract is not in the published docs. Integrators have to read the Zod schema.

Reproduce: open https://blueos.cloud/cockpit/docs/latest/usage/advanced/ and search for extras, `extras.cockpit`, `ExtrasJson`, or joystick suggestions. Nothing. The only spec is `ExtrasJsonSchema` in `src/libs/blueos/schemas.ts` (widgets, actions, optional `joystickSuggestions`; `targetCockpitApiVersion` is required on the schema and then unused at runtime). Discovery is `GET /helper/v1.0/web_services` then a fetch of whatever path `metadata.extras.cockpit` names, in `src/libs/blueos.ts`.

#2317 is about missing widget options and iframe-only widgets, not about writing this down.

Start with the published Cockpit advanced usage page, then read ExtrasJsonSchema in src/libs/blueos/schemas.ts and the discovery flow in src/libs/blueos.ts. Document the metadata.extras.cockpit path, ExtrasJson fields, and how Cockpit retrieves the file; done means the manifest contract and joystick suggestions are discoverable in the published docs.

Written by the indexing model from the issue text.