Add VitePress user guide

[brandonpayton]

Summary

• add a VitePress Kandelo guide covering the current browser UI, browser-app integration, VFS images, publishing, API stability, and troubleshooting
• deploy the guide from the existing docs workflow to https://automattic.github.io/kandelo/guide/ while preserving API docs at /api/
• update public URLs and copy so VFS images are central and the old URL-folding tagline is reserved for the docs dream section

Testing

• VITEPRESS_BASE=/kandelo/guide/ npm run docs:build
• git diff --check
• ruby -e "require 'yaml'; YAML.load_file('.github/workflows/docs.yml'); puts 'ok'"
• cd host && npm run typecheck

Note

• Local cd host && npx typedoc still fails on existing host TypeScript/Typedoc errors, so the updated docs workflow only runs API docs steps when host docs inputs change. Guide-only changes are deployed independently within the same workflow.