document support for reservation config in profiles.yml

[dejii]

What are you changing in this pull request and why?

This PR adds documentation for the new reservation config option in BigQuery profiles, introduced in dbt-labs/dbt-adapters#1229

BigQuery now supports specifying a reservation at query runtime, and this change allows dbt users to optionally configure a reservation in their profiles.yml.

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.).
• This documentation update is contingent on the acceptance of feat(bigquery): Add support for flexible job reservations dbt-adapters#1229

[welcome]

Hello!👋 Thanks for contributing to the dbt product documentation and opening this pull request! ✨
We use Markdown and some HTML to write the dbt product documentation. When writing content, you can use our style guide and content types to understand our writing standards and how we organize information in the dbt product docs.
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Project	Deployment	Preview	Updated (UTC)
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[matthewshaver]

Hi @dejii, as the attached issues are still open, what we'll do is add our icebox label to this PR and re-open it once the changes you've proposed in the product are merged. Thank you!