Add 2.x → 3.0 migration guide

[liquidsec]

Summary

• New docs/migration/3.0_breaking_changes.md covering CLI/targeting, flags, presets, modules (incl. the http name reassignment), events (VULNERABILITY removal, dict-backed URLs, no confidence attr), config renames, DNS/HTTP engine rewrite, Poetry→uv, Python ≥3.10
• Added a Migration section to mkdocs.yml and the README docs TOC
• Added an upgrade callout near the top of the README pointing at the guide

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90%. Comparing base (508b7f5) to head (e7b008c).
⚠️ Report is 15 commits behind head on docs-update.

Additional details and impacted files

@@             Coverage Diff             @@
##           docs-update   #3128   +/-   ##
===========================================
  Coverage           90%     90%           
===========================================
  Files              450     450           
  Lines            46007   46007           
===========================================
  Hits             41259   41259           
  Misses            4748    4748           

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