docs(stage): stage table function and user defined file format.

[youngsofun]

I hereby agree to the terms of the CLA available at: https://databend.rs/dev/policies/cla/

Summary

Summary about this PR

Closes #issue

[vercel]

The latest updates on your projects. Learn more about Vercel for Git ↗︎

Name	Status	Preview	Comments	Updated
databend	✅ Ready (Inspect)	Visit Preview	💬 Add your feedback	Feb 24, 2023 at 9:25AM (UTC)

[mergify]

This pull request's title is not fulfill the requirements. @youngsofun please update it 🙏.

Valid format:

fix(query): fix group by string bug
  ^         ^---------------------^
  |         |
  |         +-> Summary in present tense.
  |
  +-------> Type: rfc, feat, fix, refactor, ci, docs, chore

Valid types:

• rfc: this PR proposes a new RFC
• feat: this PR introduces a new feature to the codebase
• fix: this PR patches a bug in codebase
• refactor: this PR changes the code base without new features or bugfix
• ci: this PR changes build/testing/ci steps
• docs: this PR changes the documents or websites
• chore: this PR only has small changes that no need to record