docs: clarify Android vector icon guidance

[Whxuan0701]

Summary

• explain that Android vector drawable resources are XML, not regular SVG files
• document the Android Studio Vector Asset Studio conversion path and density behavior
• replace the Android icon references with current official documentation links

Closes #1501

Validation

• npm test
• focused Markdown link check confirms all four Android documentation URLs are present
• curl -L check returned HTTP 200 for all four official Android URLs
• git diff --check

Known environment limitation

npm run build completed the asset generation steps but could not run the Jekyll stage because the local environment does not have the jekyll executable (bundler: command not found: jekyll).