chore: update pull_request_template.md

[scottdover]

Summary
This adds a new item to the pull request template, reminding users to update changelogs and
update documentation for changes

[scnwwu]

I guess we talked about CHANGELOG previously. A concern is that we usually have multiple PRs ongoing and it will cause merge conflicts when they all change CHANGELOG. I do finallize the CHANGELOG upon release based on the milestone record. So I guess CHANGELOG is optional.

[scottdover]

I guess we talked about CHANGELOG previously. A concern is that we usually have multiple PRs ongoing and it will cause merge conflicts when they all change CHANGELOG. I do finallize the CHANGELOG upon release based on the milestone record. So I guess CHANGELOG is optional.

Makes sense. Updated the message accordingly :)