Docs: Reorganize nav to match Bolt JS follow-up.

[technically-tracy]

Summary

Follow-up tasks:

• Combined the two types of getting-started guides (http and socket mode), in both languages.
• Added Japanese nav category labels where possible.
• A few tweaks to side nav wording to align with Bolt JS.

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 (fda705f) to head (f5e3922).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1242   +/-   ##
=======================================
  Coverage   90.96%   90.96%           
=======================================
  Files         222      222           
  Lines        7501     7501           
=======================================
  Hits         6823     6823           
  Misses        678      678           

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[lukegalbraithrussell]

Small change, but looks good! I'm adding @seratch as a reviewer in case he wants to look over the consolidated Japanese Getting Started guide