docs: update Curvine core features, benchmarks, and architecture docs

[LJF-050]

Summary

• Update Benchmark docs:
  • Rewrite metadata performance test
  • Rewrite FIO test
  • Add Curvine small-file read performance test
• Update Key Features docs:
  • Merge and expand Unified Namespace / UFS documentation
  • Merge and expand Cache documentation
  • Add FUSE core feature documentation
• Add Architecture docs:
  • Add Curvine metadata architecture documentation
  • Add local metadata architecture image asset
• Sync all related English and Chinese docs

Verification

• Ran npm run build successfully
• Verified generated pages for updated Benchmark, Key Features, and Architecture docs