Add customPropertiesExtractor to the documents

[seratch]

Description

We forgot to add the document section for the customPropertiesExtractor, which was added in v3.8.0

• https://github.com/slackapi/bolt-js/pull/1177
• https://github.com/slackapi/bolt-js/tree/main/examples/custom-properties

What type of issue is this? (place an x in one of the [ ])

• [ ] bug
• [ ] enhancement (feature request)
• [ ] question
• [x] documentation related
• [ ] example code related
• [ ] testing related
• [ ] discussion

Requirements (place an x in each of the [ ])

• [x] I've read and understood the Contributing guidelines and have done my best effort to follow them.
• [x] I've read and agree to the Code of Conduct.
• [x] I've searched for any related issues and avoided creating a duplicate issue.