docs: bring v6.0.0 changelog to main

[mpoke]

Description

Closes: #XXXX

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Author Checklist

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please add links to any relevant follow up issues.

I have...

• included the correct docs: prefix in the PR title
• targeted the correct branch (see PR Targeting)
• provided a link to the relevant issue or specification
• reviewed "Files changed" and left comments if necessary
• confirmed all CI checks have passed

Reviewers Checklist

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I have...

• Confirmed the correct docs: prefix in the PR title
• Confirmed all author checklist items have been addressed
• Confirmed that this PR only changes documentation
• Reviewed content for consistency
• Reviewed content for spelling and grammar
• Tested instructions (if applicable)
• Checked that the documentation website can be built and deployed successfully (run make build-docs)