Add fill rate explanation to browser pools example

[mintlify]

Summary

• Added pool creation example with capacity of 50 and fill rate of 20 browsers/minute
• Added "Understanding fill rate" section explaining the calculation (50 ÷ 20 = 2.5 minutes to fill)
• Clarified behavior difference between reuse: false (replenishes at fill rate) and reuse: true (immediately available)

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Note

Low Risk
Low risk documentation-only change that adds examples and clarifies pool refill behavior; no runtime code paths are affected.

Overview
Updates the Browser Pools docs to include an explicit pool-creation example using fill_rate_per_minute, then adds an Understanding fill rate section that explains expected time-to-fill and the behavioral difference between releasing with reuse: false (refills at the configured rate) vs reuse: true (immediate reuse).

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[cursor]

Risk assessment: Very Low.

Evidence from diff:

• Only browsers/scaling/pools.mdx changed
• Change scope is documentation/examples text and code snippets only
• No production code, infrastructure, auth, permissions, schema, or config changes

Decision:

• Code review assignment is not required for this risk level
• Approving this PR

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