Update content for the notifications page

[Aviatorscode2]

This PR updates the content of the Notifications core concept page, focusing on two key sections:

• Introduction: This section establishes notifications as the core communication entity in Novu, tying together subscribers, events, workflows, and messages.
• Lifecycle: Describes the end-to-end flow from event triggering to message delivery, providing a clear mental model for developers integrating Novu.

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