Add pricing examples to docs

[JamieAbly]

Adds four worked pricing examples to help customers estimate costs for common use cases:

• Livestream chat – high-concurrency event chat with batching (Chat)
• Support chat – 1:1 customer support with consumption vs MAU comparison (Chat)
• Data broadcast – one-to-many updates with conflation (Pub/Sub)
• Realtime dashboard – many-to-few device monitoring (Pub/Sub)

Each example includes assumptions, cost summary, message breakdown, and relevant optimisation notes (batching, conflation, etc).

Checklist

• Commits have been rebased.
• Linting has been run against the changed file(s).
• The PR adheres to the writing style guide and contribution guide.

[coderabbitai]

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