Docs: reorganizes nav to match Bolt JS style

[lukegalbraithrussell]

Summary

No more basic / advanced concepts. Now they're organized by category.

Just one consolidated legacy steps from apps page now

To follow:

• combine the two types of getting-started guides (and in both languages)
• add japanese nav category labels where possible (as done in Bolt JS)

Testing

You can run it locally! I'd recommend doing npm run build and npm run serve so you can checkout both english and japanese. (npm run start only does one language at a time)

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 (42af666) to head (a2d2311).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1241   +/-   ##
=======================================
  Coverage   90.96%   90.96%           
=======================================
  Files         222      222           
  Lines        7501     7501           
=======================================
  Hits         6823     6823           
  Misses        678      678           

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