docs: add FXMacroData forex data example

[roberttidball]

Summary

• add a Jupytext tutorial showing how to adapt FXMacroData daily FX spot rows into Backtesting.py OHLCV data
• include a standard-library FXMacroData fetch helper using https://fxmacrodata.com/api/v1
• link the generated example from the documentation tutorial index

Notes

• The executed notebook cells use deterministic sample rows so docs builds do not depend on live network access or an API key.

Validation

• .\.venv\Scripts\python.exe "doc\examples\FXMacroData Forex Data.py"
• .\.venv\Scripts\jupytext.exe --test --to ipynb "doc\examples\FXMacroData Forex Data.py"
• .\.venv\Scripts\python.exe -m py_compile "doc\examples\FXMacroData Forex Data.py" backtesting\__init__.py
• git diff --cached --check
• .\.venv\Scripts\python.exe -m unittest backtesting.test._test.TestDocs.test_examples (skipped on Windows by upstream guard)