Add docs for the new jsonSort helper

[leeturner]

This PR adds the documentation for the new jsonSort helper that was added in WireMock 4.0.0-beta.21

Submitter checklist

• The PR request is well described and justified, including the body and the references
• The PR title represents the desired changelog entry
• If the change against WireMock 2 functionality (incompatible with WireMock 3),
  it is submitted against the 2.x branch
• The repository's code style is followed (see the contributing guide)

Details: Contributor Guide