docs: fix broken links

[WilliamBergamin]

Fixes broken links and anchors in the docs.

• Point links to the removed getting-started page at /quickstart (index) or creating-an-app (deployment guides, Japanese pages included)
• concepts/ai-apps → concepts/adding-agent-features
• Update node-slack-sdk migration guide links to their web-api/ and socket-mode/ paths
• Point Slack type links in custom-steps-dynamic-options at the per-type /reference/types/* pages
• Fix two node-slack-sdk OAuth anchors in the Japanese OAuth page

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Latest commit: af2aa34

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.38%. Comparing base (91c96e3) to head (af2aa34).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3098   +/-   ##
=======================================
  Coverage   94.38%   94.38%           
=======================================
  Files          43       43           
  Lines        7426     7426           
  Branches      704      704           
=======================================
  Hits         7009     7009           
  Misses        408      408           
  Partials        9        9           

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[zimeg]

Spot-checked the new anchors on creating-an-app and the OAuth page and they all resolve. Thanks for chasing these down!