Migrate documentation site to Docusaurus

[VGalaxies]

Summary

This PR migrates the HugeGraph documentation website preview to Docusaurus and cleans up the migrated site structure.

Key changes:

• Add a Docusaurus-based site configuration and validation workflow
• Preserve locale-aware navigation and language switching across Docs, Blog, Community, and Download pages
• Move Download pages out of Docs and move contribution/CLA pages into Community
• Localize non-badge external documentation images
• Move documentation images to native Docusaurus static paths under /img/...
• Remove duplicate legacy content image paths
• Remove stale download slug compatibility logic

Related Issue

Closes #103

Validation

• npm run validate
• ASF staging preview verified at https://hugegraph.staged.apache.org/
• Static image paths verified under /img/docs/... and /img/blog/...