DOC-6459 Integrate RS failover docs with client-specific stuff

[andy-stark-redis]

First step in integrating the general RS info about active-active health checks, failover, etc, with the info in the clients section.

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Note

Low Risk
Documentation-only changes that adjust cross-links between pages; low risk aside from potential broken relref/typo issues.

Overview
Updates the Active-Active client-library disaster recovery doc to point readers to the overview page for Client-side geographic failover before listing client-specific guides.

Also expands the referenced client list by adding the Lettuce failover guide link alongside existing Jedis and redis-py links.

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[github-actions]

DOC-6459

[github-actions]

Staging links:
https://redis.io/docs/staging/DOC-6459-rs-client-failover/operate/rs/databases/active-active/disaster-recovery/client-library-based/

[jit-ci]

🛡️ Jit Security Scan Results

✅ No security findings were detected in this PR

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^{Security scan by Jit}