Add Socket Mode healthcheck endpoint examples (#622)

[seratch]

This pull request provides examples in both async and sync apps for #622 question.

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

• slack_bolt.App and/or its core components
• slack_bolt.async_app.AsyncApp and/or its core components
• Adapters in slack_bolt.adapter
• Document pages under /docs
• Others

Requirements (place an x in each [ ])

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

Merging #623 (46180df) into main (6c868f3) will not change coverage.
The diff coverage is n/a.

❗ Current head 46180df differs from pull request most recent head 4d1a37d. Consider uploading reports for the commit 4d1a37d to get more accurate results

@@           Coverage Diff           @@
##             main     #623   +/-   ##
=======================================
  Coverage   92.09%   92.09%           
=======================================
  Files         170      170           
  Lines        5791     5791           
=======================================
  Hits         5333     5333           
  Misses        458      458           

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