Document Afterburner Translation

The [Afterburner page](https://github.com/humanmade/altis-cloud/blob/master/docs/afterburner.md) in the documentation does not mention the translation feature.

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## Acceptance Criteria

- [ ] The documentation mentions the translation feature of Afterburner

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## For Altis Team Use

### Ready for Work Checklist

Is this ticket ready to be worked on? See
[the Play Book Definition of Ready](https://playbook.hmn.md/play/product/definition-of-ready/)

- [x] Is the title clear?
- [x] Is the description clear and detailed enough?
- [x] Are acceptance criteria listed?
- [x] Have any dependencies been identified? (Optional)
- [x] Have any documentation/playbook changes been identified? (Optional)
- [x] Is an estimate or time box assigned?
- [x] Is a priority label assigned?
- [ ] Is this ticket added to an epic? (Optional)

### Completion Checklist

Is this ticket done? See
[the Play Book Definition of Done](https://playbook.hmn.md/play/product/definition-of-done-2/)

- [ ] Has the acceptance criteria been met?
- [ ] Is the documentation updated (including README)?
- [ ] Do any code/documentation changes meet project standards?
- [ ] Are automatic tests in place to verify the fix or new functionality?
- [ ] Or are manual tests documented (at least on this ticket)?
- [ ] Are any Playbook/Handbook pages updated?
- [ ] Has a new module release (patch/minor) been created/scheduled?
- [ ] Have the appropriate `backport` labels been added to the PR?