docs: document fiat currency conversion APIs

[Delgado74]

Summary

• Document all external APIs used for Bitcoin-to-Fiat conversion rates (closes Document sources for FIAT currency conversions #273)
• Coinbase (primary), Yadio.io (fallback/Latam), CoinGecko (JPY), Upbit (KRW)
• Explain fallback mechanism for regions where Coinbase is blocked

Changes

• Added docs/FiatCurrencyApis.md with detailed API documentation

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 21.86%. Comparing base (e8140c2) to head (e75332c).

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #276   +/-   ##
=========================================
  Coverage     21.86%   21.86%           
  Complexity      850      850           
=========================================
  Files           159      159           
  Lines         19874    19874           
  Branches       2529     2529           
=========================================
  Hits           4345     4345           
  Misses        14965    14965           
  Partials        564      564           

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