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.

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

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.83%. Comparing base (4d450aa) to head (fb687db).

Additional details and impacted files

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##           master    #1396   +/-   ##
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  Coverage   89.83%   89.83%           
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  Files         446      446           
  Lines       21371    21371           
  Branches     9999     9999           
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  Hits        19198    19198           
  Misses       2173     2173           

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[tusharpandey13]

The "Removed exports" table covers the auth-layer removals but is missing three low-level HTTP wrapper types that PR #1397 also removes from the public entrypoint:

Removed export (v6)	Replacement in v7
ResponseError	Management API calls throw ManagementError
FetchError	Management API calls throw ManagementError
JSONApiResponse	Responses return the data directly (no wrapper)

Suggest adding these rows so the guide is a complete upgrade checklist. Context: they are deleted in #1397 (src/lib/errors.ts, src/lib/models.ts), and are now documented in the README error-handling section on that branch.