docs: include packaging script steps in the maintainers guide

[zimeg]

Summary

This PR copies the steps for using scripts/build_pypi_package.sh from slack_bolt - reference - to this maintainers_guide.md for fast reference 📚

Testing

The updated steps were used to test that the build script packages the .whl!

Category

• /docs (Documents)
• maintainers_guide.md

Requirements

• 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 python3 -m venv .venv && source .venv/bin/activate && ./scripts/run_validation.sh after making the changes.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.36%. Comparing base (85f8f4b) to head (1426318).
Report is 1 commits behind head on main.

Additional details and impacted files

@@ Coverage Diff @@## main #1672 +/- ##
=======================================
Coverage 85.36% 85.36% =======================================
Files 113 113 Lines 12802 12802 =======================================
Hits 10928 10928 Misses 1874 1874 

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[WilliamBergamin]

Good catch 💯 sometimes knowing things lets them slip through 😢

Left one small suggestion on something I learn today