Fix links in PULL_REQUEST_TEMPLATE.md

[kbonney]

Summary

Updates links from old reatthedocs docs to current GH pages docs to address #479

Tests and documentation

n/a

Acknowledgement

By contributing to this software project, I acknowledge that I have reviewed the software quality assurance guidelines and that my contributions are submitted under the Revised BSD License.

[coveralls]

coverage: 81.79%. remained the same
when pulling a55ae13 on links
into f3b64d2 on main.