DOC-6430 changes to redis-py failover API

[andy-stark-redis]

The health check API has changed to require the checks to use an async method (relevant if you are implementing a custom health check). This PR notes that change in the Python failover page.

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Note

Low Risk
Low risk because this PR only updates documentation; the main risk is minor user confusion if the new async custom health-check example has typos or mismatched variable names.

Overview
Updates the redis-py geographic failover documentation to document a new MultiDbConfig option, health_check_timeout, for bounding the total time spent running a health check.

Reworks the custom health check example to match the updated async API: health checks run in an asyncio loop, check_health() must be async, and the example now imports from redis.asyncio.multidb.healthcheck and uses the new method signature.

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[github-actions]

DOC-6430

[github-actions]

Staging links:
https://redis.io/docs/staging/DOC-6430-py-failover-updates/develop/clients/redis-py/failover/

[jit-ci]

🛡️ Jit Security Scan Results

✅ No security findings were detected in this PR

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^{Security scan by Jit}

[dwdougherty]

LGTM.

[andy-stark-redis]

Thanks @dwdougherty !