[Improvement][Seatunnel-web] Document Placeholder feature

[arshadmohammad]

Documented configuration placeholder feature

Purpose of this pull request

Check list

• Code changed are covered with tests, or it does not need tests for reason:
• If any new Jar binary package adding in your PR, please add License Notice according
  New License Guide
• If necessary, please update the documentation to describe the new feature. https://github.com/apache/seatunnel/tree/dev/docs

[arshadmohammad]

@wuchunfu, could you please review and merge this PR? If there are any issues with the Chinese version of the document, could you modify and include them in your PR #229 ?