ci(pr-template): modify pr template and move location (#1877)

* ci(pr-template): modify pr template and move location

Based on https://stackoverflow.com/questions/52139192/github-pull-requests-template-not-showing, renamed and moved pr template

* improved pr template and added references as suggested by @wpbonelli

* codespell!

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,14 @@
+Checklist of items for pull request
+
+- [ ] Closed issue #xxxx
+- [ ] Referenced issue or pull request #xxxx
+- [ ] Added new test or modified an existing test
+- [ ] Formatted new and modified Fortran source files with `fprettify`
+- [ ] Added doxygen comments to new and modified procedures
+- [ ] Updated meson files, makefiles, and Visual Studio project files for new source files
+- [ ] Updated [definition files](/MODFLOW-USGS/modflow6/tree/develop/doc/mf6io/mf6ivar)
+- [ ] Updated [develop.tex](/MODFLOW-USGS/modflow6/doc/ReleaseNotes/develop.tex) with a plain-language description of the bug fix, change, feature; required for changes that may affect users
+- [ ] Updated [input and output guide](/MODFLOW-USGS/modflow6/doc/mf6io)
+- [ ] Removed checklist items not relevant to this pull request
+
+For additional information see [instructions for contributing](/MODFLOW-USGS/modflow6/.github/CONTRIBUTING.md) and [instructions for developing](/MODFLOW-USGS/modflow6/.github/DEVELOPER.md).