USWDS-Next: Add PR template

[amyleadem]

Summary

Added a PR template to the uswds-next repo.

[!Note] Right now the template is pretty basic and similar to our other PR templates, but I expect this PR template will evolve as the repo does. For example, I could imagine adding sections for any component doc requirements we might develop. For now, I didn't speculate on what we might want and went with only the sections that felt solid. We should continue to revisit this PR template as we move forward.

[!Note] This is based on the PR templates in the uswds, uswds-site, and uswds-compile PR templates

Related issue

Closes #34

Preview link

PR template in preview mode

Testing and review

• Confirm that the structure and content of the PR template makes sense for the repo
• Confirm that the PR template is in the right location in the .github directory