Build: remove docusaurus configuration files

[lukegalbraithrussell]

Summary

This PR removes the docusaurus build from this repo, moving to a centralized setup. Markdown files only from now on in the open source repos.

This PR should not be merged until these docs are live on docs.slack.dev/bolt-python. (I'm specifying this build-docs-remove-docusaurus branch in the initial build to prevent downtime).

This will take place at the same time as all the other tools repos migrations

Testing

trust me ;) - it'll also be live on the new site before merge so you can double check if you want

Category

• 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

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 (9596797) to head (45f7421).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1342   +/-   ##
=======================================
  Coverage   90.96%   90.96%           
=======================================
  Files         222      222           
  Lines        7501     7501           
=======================================
  Hits         6823     6823           
  Misses        678      678           

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