[Proposal] Refinement project and Preview/Pages support.

[Koy Zhuang]

Summary

After we do the refinement #2316 and remove /index.html, the project structure changes.
Negative Effects:

• It makes the vercel-Preview doesn't work.
• We only keep the /docs/index.html with cdn resources, which is inconvenient for local dev.

---

Solution:

As-is:

• We generated a /docs/preview.html for those requirements with a redirect config to make preview works.

Proposal:

• solution1:

Keep current changes and do refine:

Generate the preview.html into /index.html.

Benefit:
We could remove the redirect config for preview now.

Related PR: #2322

• solution2:

Keep project for dev and contribution and leave those adoption in CI or other stuff.

Change the /docs/index.html imports from CDN to local.
Benefit:
We could more easily dev and test for project.
We don't need any project level adaption for Pages/Preview stuff, which out of the project's codebase scope.

Related PR (WIP): #2317