docs: add Valkey storage and vector database documentation

[atao2004]

Summary

Adds documentation for the Valkey integration (storage adapter + vector database), companion to agno-agi/agno#8141.

Storage adapter (database/providers/valkey/)

• Overview with Docker setup, basic usage, and configuration params
• Usage examples: agent, team, workflow storage

Vector database (knowledge/vector-stores/valkey/)

• Overview with setup (requires valkey/valkey-bundle for search module)
• Code example with Knowledge + Agent
• Metadata filtering documentation (dict-based on indexed TAG fields)
• Configuration params table

Notes

• database/valkey.mdx is a leftover from an incorrect initial placement — should be deleted (the correct file is at database/providers/valkey/overview.mdx)
• docs.json navigation entries need to be added for the Valkey pages (happy to do this if you point me to the right section structure)
• No <Snippet> param files created yet — used inline tables instead. Happy to refactor to snippets if preferred.

Checklist

• Follows existing doc structure (mirrors Redis docs)
• Code examples use correct imports from the Valkey adapter
• Docker instructions included
• Configuration parameters documented