feat(community): add s3-vectors-memory plugin

[nihilg]

Summary

Adds community plugin page for s3-vectors-memory — a long-term semantic memory plugin for Strands Agents backed by Amazon S3 Vectors.

What it does

• Summarizes conversations at session end and stores them as searchable vectors
• Injects relevant summaries into the system prompt on the next session
• Supports single-tenant and multi-tenant (SaaS) deployments with IAM-enforced
  tenant isolation via the Token Vending Machine pattern

Checklist

• Doc file added at community/plugins/s3-vectors-memory.mdx
• navigation.yml updated
• Follows the structure of existing community plugin pages
• Package published to PyPI: https://pypi.org/project/strands-s3-vectors-memory/