Add Beta Threat Intelligence API docs (#21)

* Add Beta Threat Intelligence API docs

Add a new beta API documentation page (src/content/docs/API solutions/Threat Intelligence API/beta.md) describing the Beta tier endpoints. The new doc covers npm ecosystem support, resource-based nested response shapes, optional advisory body (?include=details), offset and cursor pagination, endpoints (/all, /latest, /product, /batch), examples (curl/Node/PHP), error handling, and migration notes from v2. Also update overview.md to advertise and link to the new Beta Threat Intelligence API docs.

* url update

Author: mariojgt