Add issue and pull request templates (#2560)

.github/pull_request_template.md

@@ -0,0 +1,24 @@
+<!--
+If you are a first-time contributor to OpenMC, please have a look at our
+contributing guidelines:
+https://github.com/openmc-dev/openmc/blob/develop/CONTRIBUTING.md
+-->
+
+# Description
+
+Please include a summary of the change and which issue is fixed if applicable. Please also include relevant motivation and context.
+
+Fixes # (issue)
+
+# Checklist
+
+- [ ] I have performed a self-review of my own code
+- [ ] I have run [clang-format](https://docs.openmc.org/en/latest/devguide/styleguide.html#automatic-formatting) on any C++ source files (if applicable)
+- [ ] I have followed the [style guidelines](https://docs.openmc.org/en/latest/devguide/styleguide.html#python) for Python source files (if applicable)
+- [ ] I have made corresponding changes to the documentation (if applicable)
+- [ ] I have added tests that prove my fix is effective or that my feature works (if applicable)
+<!--
+While tests will automatically be checked by CI, it is good practice to
+ensure that they pass locally first. See instructions here:
+https://docs.openmc.org/en/latest/devguide/tests.html
+-->