Specify best practices for generator PRs

[MIchaelMainer]

https://github.com/microsoftgraph/MSGraph-SDK-Code-Generator/blob/dev/.github/pull_request_template.md

Also capture best practices for how to generate code files based on these changes, and how to best perform and structure the resulting PRs from the changes made in the generator repo. We will want to add to the PR templates and our guidance OneNote
AB#8473