change: update PR template with items about tests, regional endpoints, and API docs

[laurenyu]

Description of changes:

1. We recently had some code changes that weren't agnostic to the region or AWS account running the integ tests, which caused issues with our release and test processes. Adding this extra checklist item to encourage people to remember to look out for that.
2. Often there's manual testing that's happening but not getting documented. I'm hoping that adding another prompt will encourage people to show how they've tested their changes, especially if it was more involved than simply running the test suite.
3. We should be using regional endpoints for S3 and STS, even though boto3 does not give us this behavior automatically like it does with other services. We've talked before about having a more automated check, but in the meantime a checklist item is probably the quickest mechanism.

Merge Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your pull request.

• I have read the CONTRIBUTING doc
• I used the commit message format described in CONTRIBUTING
• I have added tests that prove my fix is effective or that my feature works (if appropriate)
• I have updated any necessary documentation (if appropriate)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

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