Update the documents for v3.9 release

[seratch]

Summary

This pull request updates the document pages for v3.9 release. The changes are:

• Add "Sign in with Slack" guide in the OAuth module page
• Add RetryHandler guide in the API client module pages

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 ./docs.sh?)
• /docs-src-v2 (Documents, have you run ./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

Merging #1087 (63fcdd7) into main (daff025) will increase coverage by 0.12%.
The diff coverage is n/a.

❗ Current head 63fcdd7 differs from pull request most recent head 8591765. Consider uploading reports for the commit 8591765 to get more accurate results

@@            Coverage Diff             @@
##             main    #1087      +/-   ##
==========================================
+ Coverage   86.07%   86.19%   +0.12%     
==========================================
  Files         110      110              
  Lines        9932     9932              
==========================================
+ Hits         8549     8561      +12     
+ Misses       1383     1371      -12     

Impacted Files	Coverage Δ
slack_sdk/scim/v1/client.py	93.75% <0.00%> (ø)
slack_sdk/webhook/client.py	94.59% <0.00%> (ø)
slack_sdk/web/base_client.py	89.55% <0.00%> (ø)
slack_sdk/audit_logs/v1/client.py	91.20% <0.00%> (ø)
slack_sdk/scim/v1/async_client.py	94.20% <0.00%> (ø)
slack_sdk/webhook/async_client.py	92.23% <0.00%> (ø)
slack_sdk/web/async_base_client.py	98.33% <0.00%> (ø)
slack_sdk/audit_logs/v1/async_client.py	89.16% <0.00%> (ø)
slack_sdk/http_retry/builtin_handlers.py	92.10% <0.00%> (+31.57%)	⬆️

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[filmaj]

Looks great! Thanks for writing this up

[seratch]

Thanks for your review!

Once we release v3.9.0 to the PyPI registry, we will merge this PR and also will generate the API docs with the latest code