docs: Add a Reference design document page to provide example designs using autoware

[chishengshih]

Description

One new tab is added for reference design. The page will collect the design summary for projecting using Autoware and the design guideline for Level 4 vehicles using Autoware.

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[stale[bot]]

This pull request has been automatically marked as stale because it has not had recent activity.

[susu3621]

The document looks great, is there anyone following to get the PR merged?