Source freshness

[nataliefiann]

What are you changing in this pull request and why?

I have created this PR following this Git issue #4408 raised by Colin T to update the docs to clarify deferral content.

Initially there was a discussion on creating a guide but since the docs already have some info on source freshness, I opted to update the About dbt source command doc and Add sources to your DAG doc.

Closes: #4408

Checklist

• I have reviewed the Content style guide so my content adheres to these guidelines.
• The topic I'm writing about is for specific dbt version(s) and I have versioned it according to the version a whole page and/or version a block of content guidelines.
• I have added checklist item(s) to this list for anything anything that needs to happen before this PR is merged, such as "needs technical review" or "change base branch."
• The content in this PR requires a dbt release note, so I added one to the release notes page.

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🚀 Deployment available! Here are the direct links to the updated files:

• https://docs-getdbt-com-git-nfiann-sourcefreshness-dbt-labs.vercel.app/docs/build/sources
• https://docs-getdbt-com-git-nfiann-sourcefreshness-dbt-labs.vercel.app/reference/commands/source

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