docs/contributing: how to find undocumented new region values

[PatMyron]

Recently left Amazon, so documenting how I found undocumented new region account and hosted zone ID values for the last couple AWS regions:
#12967
#13061
#12976
#21337
#22295
so someone else at Amazon can carry the torch for new AWS regions :)

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[breathingdust]

LGTM @PatMyron! 🚀 Thanks for your help and good luck in whatever you decide to do next! 👍

[github-actions]

This functionality has been released in v3.58.0 of the Terraform AWS Provider. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading.

For further feature requests or bug reports with this functionality, please create a new GitHub issue following the template. Thank you!

[github-actions]

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