Add customPropertiesExtractor to the documents

[seratch]

Description

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

• Fix #759 #1109 #1110 by adding custom properties in ReceiverEvent and Context objets #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
• documentation related
• example code related
• testing related
• discussion

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

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

[mlauter]

@seratch do you imagine this going in the Customizing a receiver section or as a standalone docs section? Currently that customizing a receiver section just points folks to the source for ExpressReceiver for more info, and doesn't list or document receiver options.

Happy to take a stab at a PR for this -- just wanted to check in about what folks had in mind!

[mlauter]

@seratch just wanted to check what you had in mind here^

[seratch]

Hi @mlauter, thanks for your interest. I was going to simply add it to https://slack.dev/bolt-js/reference#receiver-options If you're interested in working on it, we are happy to have your contribution!

[mlauter]

Hi @mlauter, thanks for your interest. I was going to simply add it to https://slack.dev/bolt-js/reference#receiver-options If you're interested in working on it, we are happy to have your contribution!

got it that makes sense - lol, i've been reading these docs frequently for a long time now and never noticed the reference section in the sidebar, was just going to the source code 🤦 . I wonder if there's a way to make it more noticeable, it sort of gets lost in all the concept links in the sidebar.