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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Name	Status	Preview	Updated (UTC)
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[mirnawong1]

hey @nataliefiann , here's a checklist of things i recommend you check out and summarize my first pass in reviewing:

• best to create and use snippets if you want to use content in more than one page.
• for docs links — use relative paths, not absolute
• i might be misunderstanding but don’t really get what the ‘Build models based on source freshness’ section is trying to communicate? is it trying to tell us about how source freshness works? yr about tests? the header mentions its about building models based on source freshness but i dont really understand how a user would do that based on this info?
• sugg your pr has a tech review first before a docs review so you can finalize any technical content before it reaches docs team