Adding tab borders

[matthewshaver]

What are you changing in this pull request and why?

Enhancing tabs:

• Places borders around individual tabs
• Sets a unique background color for clicked and hovered tabs
• Puts a border around the entire table area so it's clear where tabbed content ends

Closes #3889

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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