COMDOX-1796 GraphQL for Per source availability

[jhadobe]

Purpose of this pull request

Documents the new sourceAvailability GraphQL query (ACCS-933), which returns per-source inventory availability for one or more SKUs, scoped to the current sales channel's stock. Validated against PR #1729.

Covers the query syntax and arguments (skus, source_codes, only_in_stock), the admin opt-in toggle, available_qty masking via the Only X left Threshold setting, and error responses. Also links the new page from the Product queries index.

whatsnew
Added sourceAvailability query documentation for reading per-source inventory availability by SKU, scoped to the current sales channel.

[github-actions]

🔍 Linter Report

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                     LINTER REPORT
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Generated: 2026-09-02T21:10:26.837Z
Mode: Full Linting (all rules + dead links check)
Target Directory: /home/runner/work/commerce-webapi/commerce-webapi
Skipped Rules:
  - check-frontmatter → src/pages/includes
  - missing-heading → src/pages/includes

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Files to process: 2


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                        SUMMARY
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  📁 Files processed:    2
  📄 Files with issues:  0
  ❌ Total errors:       0
  ⚠️  Total warnings:     0
  📋 Total issues:       0

Result: ✅ PASSED - All files passed linting successfully!

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