docs: add v7 migration guide (v6 → v7)

[tusharpandey13]

Summary

Adds v7_MIGRATION_GUIDE.md, the release-level upgrade guide for 6.x → 7.x, following the conventions of the existing v5/v6 guides. Kept separate from the Authentication API deep-dive guide (#1395), which it references rather than duplicates.

v7 makes node-auth0 a Management-API-only SDK. This guide is the umbrella that covers the full breaking surface of that release:

• Authentication API removed from the main entrypoint — points to AUTH_MIGRATION_GUIDE.md for the method-by-method rewrite.
• Removed-exports table — AuthenticationClient, UserInfoClient, AuthApiError, AuthenticationClientOptions, IDTokenValidateOptions, IdTokenValidatorError, TokenSet, SUBJECT_TOKEN_TYPES, UserInfoResponse, UserInfoError, each mapped to its v7 replacement.
• Management-side breaking changes not in scope of the auth guide: mTLS requires an explicit fetch; mTLS + client-assertion mutually exclusive; bare-hostname domain validation; token-acquisition failures throw ManagementError; uuid dependency removed.
• Legacy entrypoint documented as a stopgap.

Also links the new guide from the README documentation list.

Why a separate guide (not a rename of AUTH_MIGRATION_GUIDE)

AUTH_MIGRATION_GUIDE.md (#1395) is a topic guide for the auth-layer rewrite and explicitly scopes out the Management API. v7 is a superset: it also covers the Management-side breaks and the export-surface removals, matching how the v6 guide is the release umbrella. Renaming the auth guide would silently drop two-thirds of the v7 breaking surface.

Base branch

Targets feat/auth-separation-v6 (the v7 [BREAKING] release branch, #1390) so the guide ships with the release it documents.

Related PRs

• Code (auth-layer removal): [BREAKING] feat!: remove Authentication layer #1390
• Authentication Migration Guide (auth-layer rewrite detail): docs: add Authentication API migration guide (auth0-auth-js / auth0-server-js) #1395
• Migration skill: feat(auth0): node-auth0 → auth0-auth-js/server-js migration skill agent-skills#176

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.73%. Comparing base (f9d6413) to head (dd07f02).

Additional details and impacted files

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##           master    #1396    +/-   ##
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  Coverage   89.73%   89.73%            
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  Files         441      441            
  Lines       20799    20799            
  Branches    10146     9809   -337     
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  Hits        18663    18663            
  Misses       2136     2136            

Flag	Coverage Δ
alltests	89.73% <ø> (ø)

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