docs: filter against bot_id in listener middleware example

[zimeg]

Summary

This PR updates the listener middleware example to filter out bot messages according to a bot_id value to match the changes of slackapi/bolt-js#2610:

In classic Slack apps, the bot_message event is sent when a message is sent to a channel by an integration "bot". It is like a normal user message, except it has no associated user. With current Slack apps, also known as granular bot permissions (GBP) apps, you can detect if a message was sent by a bot by the presence of the bot_id and bot_profile fields in the event payload.

🔗 https://api.slack.com/events/message/bot_message

Testing

The updated examples can be used as a proof of concept!

Category

• Document pages under /docs

Requirements

Please read the Contributing guidelines and Code of Conduct before creating this issue or pull request. By submitting, you are agreeing to those rules.

• 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 run ./scripts/install_all_and_run_tests.sh after making the changes.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.96%. Comparing base (88ddc7c) to head (4f10adb).
Report is 1 commits behind head on main.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@ Coverage Diff @@## main #1339 +/- ##
=======================================
Coverage 90.96% 90.96% =======================================
Files 222 222 Lines 7501 7501 =======================================
Hits 6823 6823 Misses 678 678 

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[lukegalbraithrussell]

かんぱい

[zimeg]

@lukegalbraithrussell I so much appreciate the review - let's now merge this 👾 ✨