docs: clarify private deployment requirements

[HynLcc]

What changed

• Clarify that switching an existing Space to a database tenant briefly pauses writes.
• Document Vercel and Teable Infra as App Builder deployment engines.
• Limit the public-access requirement and test to Vercel deployments.
• Keep English and Chinese help-center pages aligned.

Why

The previous wording treated public access as a requirement for every self-hosted App Builder deployment, while the current UI only requires it for Vercel.

Validation

• git diff --check
• mintlify validate