docs: write comprehensive READMEs for sovereign_debt_xl and sovereign_debt_py

Both READMEs now document every function (73 total across 18 modules) with:
- Full description of what the function does and the underlying methodology
- Parameter table (name, type, description)
- Return value description
- Concrete usage example (Excel formula syntax for xl; Python code for py)
- sovereign_debt_xl README also covers installation, PyXLL setup, array formula guidance, and error handling
- sovereign_debt_py README also covers installation, quick start, and how return types work

Co-authored-by: zachessesjohnson <168567202+zachessesjohnson@users.noreply.github.com>

Author: Copilot