docs: update homepage hero and feature cards

[xiami762]

Summary

• Rename the homepage hero from "Flocks 文档" to "Flocks".
• Reorder and rename the six homepage feature cards to match the requested product capability areas.
• Point the security device card to the device management documentation.

Validation

• git diff --check
• Verified the homepage frontmatter contains the expected hero name and feature-card title order.

Note

• npm run build currently fails because VitePress cannot resolve vue/server-renderer from docs/index.md in this checkout; this appears to be an existing dependency-resolution issue unrelated to this content change.