Add API Index Component for default theme

[sheepbox8646]

When I write the API References for my opensource project, it maybe get a mess if you use common markdown. I don't know how to solve it until I discover Microsoft's mocano editor's documentation. It's API Documentation is so tidy that I can find the API I want to find with a second of sight. So I want to write a similar page for VitePress.

Preview

The test file is in tests/e2e/api-content.md

[sheepbox8646]

Emm...Maybe the color was a little ugly, please comment tips that I can fix