chore: align README pattern

[WilliamBergamin]

Summary

This PR updates the readme to follow the same pattern found in bolt, the sdk and the hooks projects

Preview

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

• slack_sdk.web.WebClient (sync/async) (Web API client)
• slack_sdk.webhook.WebhookClient (sync/async) (Incoming Webhook, response_url sender)
• slack_sdk.socket_mode (Socket Mode client)
• slack_sdk.signature (Request Signature Verifier)
• slack_sdk.oauth (OAuth Flow Utilities)
• slack_sdk.models (UI component builders)
• slack_sdk.scim (SCIM API client)
• slack_sdk.audit_logs (Audit Logs API client)
• slack_sdk.rtm_v2 (RTM client)
• /docs-src (Documents, have you run ./scripts/docs.sh?)
• /docs-src-v2 (Documents, have you run ./scripts/docs-v2.sh?)
• /tutorial (PythOnBoardingBot tutorial)
• tests/integration_tests (Automated tests for this library)

Requirements (place an x in each [ ])

• 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 84.86%. Comparing base (b464941) to head (54f94b3).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1513      +/-   ##
==========================================
- Coverage   84.91%   84.86%   -0.05%     
==========================================
  Files         112      112              
  Lines       12473    12473              
==========================================
- Hits        10591    10585       -6     
- Misses       1882     1888       +6     

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