docs: add CURRENCY_WHITELIST.md contributor guide

[tebney]

Summary

Adds docs/CURRENCY_WHITELIST.md — a contributor-focused guide explaining how tokens are added to and removed from the currency whitelist.

Closes #1549

What's included

• Explains the empty-list / allow-all mode and when it becomes restrictive
• Documents all 10 contract entrypoints with auth requirements and return types
• Covers the two enforcement points (store_invoice/upload_invoice and place_bid)
• Describes the two-layer auth model (storage check + require_auth)
• Provides a concrete test helper pattern using soroban_sdk primitives (no std::)
• Step-by-step production add/remove workflow
• Cross-links to docs/contracts/currency-whitelist.md, source, and tests

Acceptance criteria checklist

• Matches the summary (how tokens are added/removed from the whitelist)
• Linked from README.md Documentation section
• No compiled examples (doc-only change; no code added to contract crate)
• CI will skip Soroban contract build for this docs-only PR (paths-filter in ci.yml)
• References Closes #1549

Testing

Docs-only change — no contract source modified. The CI paths-filter skips the Soroban build/test steps for PRs that don't touch quicklendx-contracts/.

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