B2 SDK docs: add Event Notifications usage and API documentation #596

Description

doc/source/index.rst only announces that Event Notifications are in Private Preview. Meanwhile, b2sdk.v3 exports NotificationRule, NotificationRuleResponse, EVENT_TYPE, and conversion helpers, and the API and bucket layers expose notification operations. There is no SDK guide explaining how preview access affects availability, which operations are supported, what a rule contains, or how response objects should be interpreted.

Steps to reproduce

from b2sdk.v3 import NotificationRule, NotificationRuleResponse

Search doc/source for an Event Notifications guide or API page.

Expected behavior

The feature should have documentation appropriate to its release status. It should state the preview-access requirement and stability expectations, document the required permissions, show a minimal create/list/update/delete workflow, explain event types and rule fields, cover common errors, and link the intended public types and methods to API references.

Actual behavior

Users see a preview announcement and an external blog link but cannot learn how the Python SDK exposes the feature. This makes exported functionality look unsupported or forces users to infer usage from implementation code and tests.

Environment

• Python version: Any supported version
• b2sdk version: master at f3ab7b71
• OS: Any