Added documentation for configuring the execution_project on a per-model basis

[bordway-shop]

What are you changing in this pull request and why?

Documentation related to the changes made in the BigQuery dbt-adapter code.

Checklist

• The changes in this PR meet the docs style guide/fundamentals required for all content.
• Applied the proper versioning rules if the content is for specific dbt version(s): (version a whole page or version a block of content
• The content in this PR requires a dbt release note, so I added one to the release notes page.).

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