[feat i2g] add documentation for migration tool

[shuqz]

Issue

Related to #4728 — this PR extracts and restructures the migration documentation. Should be merged after #4728.

Description

Restructures the Ingress-to-Gateway migration documentation into a 3-page layout for clarity:

1. Migration Guide (migrate_from_ingress.md) — end-to-end 6-step walkthrough (translate → preview → apply → verify → shift traffic → cleanup)
2. Migration Tool (lbc_migrate_reference.md) — full CLI reference (flags, annotation support table, output formats, IngressGroup handling, known differences)
3. Migration Console (in_cluster_console.md) — console UI walkthrough (diff classification, export, RBAC, troubleshooting)

Also adds diagrams generated with matplotlib:

• 6-step migration flow overview
• Dry-run verification workflow
• Service reuse architecture
• Traffic shifting strategies comparison

Documentation only — no code changes.

Checklist

• Added tests that cover your change (if possible)
• Added/modified documentation as required (such as the README.md, or the docs directory)
• Manually tested
• Made sure the title of the PR is a good description that can go into the release notes

BONUS POINTS checklist: complete for good vibes and maybe prizes?! 🤯

• Backfilled missing tests for code in same general area 🎉
• Refactored something and made the world a better place 🌟

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: shuqz

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Details

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• OWNERS [shuqz]

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 56.57%. Comparing base (22cdb42) to head (26bf955).
⚠️ Report is 45 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4755      +/-   ##
==========================================
+ Coverage   56.09%   56.57%   +0.47%     
==========================================
  Files         388      390       +2     
  Lines       30932    31138     +206     
==========================================
+ Hits        17352    17616     +264     
+ Misses      12566    12496      -70     
- Partials     1014     1026      +12     

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