docs: add customer groups and wholesale pricing guides

[mbates]

Summary

Documentation follow-up for #64 (customer groups + pricing rules).

• New guide: docs/guides/core/customer-groups.md — full coverage of square.customerGroups (CRUD, list pagination, membership) plus the wholesale workflow.
• docs/guides/core/catalog.md — new Wholesale Pricing section covering:
  • The product set + discount + pricing rule trio
  • createWholesalePricing() (composite atomic helper)
  • Manual rule construction for advanced cases
  • Time-bounded rules with TIME_PERIOD (RFC 5545 RRULE) and date-bounded rules via validFromDate/validUntilDate
• README.md — service table now includes customerGroups and checkout; pricing rules called out under catalog.
• docs/README.md — linked the new guide.
• CLAUDE.md — added a Documentation section establishing the rule that public API changes require docs in the same PR (or an immediate follow-up like this one).

Test plan

• Markdown only — no code or tests touched
• All internal links resolve to real files

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.06%. Comparing base (756b613) to head (dd303cf).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #65   +/-   ##
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  Coverage   97.06%   97.06%           
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  Files          18       18           
  Lines         955      955           
  Branches      277      277           
=======================================
  Hits          927      927           
  Misses          1        1           
  Partials       27       27           

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[github-actions]

🎉 This PR is included in version 1.11.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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