Migrate TiDB Cloud Premium docs for preview site to docs.pingcap.com

[lilin90]

Background:

Before public preview, we put Premium docs only in a preview doc site. When Premium enters public preview, we need to publish its documentation on the public documentation site docs.pingcap.com.

When: On the day of public preview.

Preparation and dependencies:

• All feature docs ready @lilin90 @wildpcww
• Developer guides ready @Oreoxmt
• Onsite search ready @shhdgit
• Updating all affected doc links in console @shhdgit