chore: move PR template to .github/ dir (#4060)

## What type of PR is this? (check all applicable)

- [x] Refactor

## Have you discussed this change with the InvokeAI team?
- [x] No, because it's pretty minor

      
## Have you updated all relevant documentation?
- [x] No


## Description

This PR just moves the PR template to within the `.github/` directory
leading to a overall minimal project structure.

## Added/updated tests?

- [x] No : because this change doesn't affect or need a separate test