Document new helpers and extension lifecycle

[leeturner]

This PR documents the new arrayAdd, arrayRemove, arrayJoin and val helpers along with the new start() and stop() extension lifecycle methods

Submitter checklist

• [x] The PR request is well described and justified, including the body and the references
• [x] 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
• [x] The repository's code style is followed (see the contributing guide)

Details: Contributor Guide