Add PR reminder to update docs

Author: duckontheweb

.github/pull_request_template.md

@@ -8,5 +8,6 @@
 
 - [ ] Code is formatted (run `pre-commit run --all-files`)
 - [ ] Tests pass (run `scripts/test`)
+- [ ] Documentation has been updated to reflect changes, if applicable
 - [ ] This PR maintains or improves overall codebase code coverage.
 - [ ] Changes are added to the [CHANGELOG](https://github.com/stac-utils/pystac/blob/develop/CHANGELOG.md). See [the docs](https://pystac.readthedocs.io/en/latest/contributing.html#changelog) for information about adding to the changelog.