[sources.md] Added google sheet ingestion#12

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kennethrioja wants to merge 2 commits intoElixirTeSS:mainfrom

kennethrioja:gsheet

Commits on Oct 20, 2025

docs(sources): added google sheet ingestion
kennethrioja
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Commits on Oct 21, 2025

Refined instructions for Google Sheets ingestion
PhilReedData
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