docs: add PULL_REQUEST_TEMPLATE.md.

.github/PULL_REQUEST_TEMPLATE.md

+#### What this PR does / why we need it?
+
+#### Summary of your change
+
+#### Please indicate you've done the following:
+
+- [ ] Made sure tests are passing and test coverage is added if needed.
+- [ ] Made sure commit message follow the rule of [Conventional Commits specification](https://www.conventionalcommits.org/).
+- [ ] Considered the docs impact and opened a new docs issue or PR with docs changes if needed.