Document optional external sentiment data columns

[alexander-schneider]

Summary

• Document how to attach external sentiment data as an extra strategy DataFrame column
• Show an optional Adanos Market Sentiment API loader using only the standard library and existing pandas dependency
• Keep the example runnable without an API key by falling back to sample sentiment values

Validation

• .venv/bin/python -m py_compile 'doc/examples/Quick Start User Guide.py'
• .venv/bin/jupytext --test --to ipynb 'doc/examples/Quick Start User Guide.py'
• diff <(strip_yaml 'doc/examples/Quick Start User Guide.py') <(.venv/bin/jupytext --to py --output - 'doc/examples/Quick Start User Guide.ipynb' | strip_yaml)
• .venv/bin/python -m backtesting.test (76 tests, 1 skipped)

[kernc]

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Otherwise a duplicate of #39.