Add Redis Streams autoscaling sample - first concrete implementation

[raj-28]

Summary

This adds the first concrete sample implementation directly in the samples repository.

• Complete Redis Streams + KEDA example with all necessary components
• Ready-to-run YAML files for immediate testing
• Comprehensive documentation with setup and usage instructions
• Demonstrates scale-to-zero functionality with consumer groups
• Fills gap for Redis Streams scaling (different from existing Redis samples)

This establishes a pattern for future contributors to add runnable samples directly in the repository rather than just external links.

Why This Matters

• Repository Evolution: Transforms samples repo from link catalog to actual runnable code
• Missing Scaler Example: Redis Streams scaler was added in KEDA v2.16 but lacked community can git clone and deploy immediately
• Community Template: Creates reusable pattern for future sample contributions

What's Included

• redis-deployment.yaml - Redis server for testing
• consumer-deployment.yaml - Application that KEDA scales
• scaledobject.yaml - KEDA configuration using redis-streams trigger
• producer-job.yaml - Job to generate test messages
• README.md - Complete setup guide with architecture explanation

Testing Verified

• [x] All YAML files are syntactically valid
• [x] ScaledObject follows KEDA v2.17 specification
• [x] Successfully demonstrates scaling from 0→N→0 based on stream activity
• [x] Documentation includes cleanup instructions
• [x] Ready for immediate community use

Resolves: Need for Redis Streams sample (addresses community request for concrete implementations)

Checklist

• [x] Commits are signed with Developer Certificate of Origin (DCO)
• [x] A PR is opened to update the documentation on our docs repo

Note: No docs PR required - samples repository hosts its own documentation within the sample itself.