docs: fix broken links in v8 migration guides

[WilliamBergamin]

Summary

This pull request fixes the last two broken links that the docs.slack.dev build reports.

• migration/socket-mode/migrating-socket-mode-package-to-v3.md: ./web-api-v8-migration.md doesn't exist. It now points at ../web-api/migrating-web-api-package-to-v8.md.
• migration/web-api/migrating-web-api-package-to-v8.md: the RequestConfig section linked to requestinterceptor-and-requestinterceptor-type-removed with no # and the wrong slug. It now links to #weve-removed-the-requestinterceptor-and-requestinterceptor-types.

The docs.slack.dev build is about to start failing on broken links, so these need to be fixed first. Follow-up to #2759.

Testing

• Built docs.slack.dev locally with these files and broken links set to throw: the build succeeds with no broken links, anchors, or markdown links reported.
• On the v8 web-api migration page, click "that has also been removed" under RequestConfig and confirm it jumps to the requestInterceptor section.
• On the socket-mode v3 page, click "web-api v8 migration guide" and confirm it opens the web-api v8 migration page.

Requirements

• 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.

🤖 Generated with Claude Code

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Latest commit: cd85761

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.36%. Comparing base (e02e7fe) to head (cd85761).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2768      +/-   ##
==========================================
- Coverage   89.37%   89.36%   -0.01%     
==========================================
  Files          65       65              
  Lines       10525    10525              
  Branches      478      478              
==========================================
- Hits         9407     9406       -1     
- Misses       1086     1087       +1     
  Partials       32       32              

Flag	Coverage Δ
cli-hooks	89.32% <ø> (ø)
cli-test	89.32% <ø> (ø)
logger	89.32% <ø> (ø)
oauth	89.32% <ø> (ø)
socket-mode	89.32% <ø> (ø)
web-api	89.32% <ø> (ø)
webhook	89.32% <ø> (ø)

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