3 7 0 release updates

[leeturner]

This PR updates the docs with the outstanding 3.7.0 updates:

• Adds the max template cache entry size default
• Info on the new webhook sub event infomation
• Adds the new --preserve-user-agent-proxy-header config options

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
• [x] 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