docs: add BigQuery OLAP (live connector) page

[royendo]

• New docs/.../olap/bigquery.md mirroring the Snowflake OLAP page: live-vs-data-source tip, service account + ADC auth, max_bytes_billed cost controls, and a metrics view example mapping database→project, database_schema→dataset, model→table.
• Added BigQuery to the OLAP engines index (olap.md) heading list and ConnectorIcon grid using the existing Logo-BigQuery.svg.

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!

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Developed in collaboration with Claude Code