add issue and PR templates

.github/issue_template.md

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+<!--
+Thank you for opening an issue for asdf!
+
+Your feedback is crucial to maintaining this open source project.
+
+If this is a feature request or discussion question please add to
+or create a new discussion:
+
+https://github.com/asdf-format/asdf/discussions
+-->
+
+# Description of the problem
+
+<!-- If an Exception, Warning or some other error occurred please include
+the error message and full traceback -->
+
+# Example of the problem
+
+<!-- Please include an example that reproduces the problem or describe why
+an example isn't provided. -->
+
+# System information
+
+asdf version:
+python version:
+operating system:

.github/pull_request_template.md

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+<!---
+Thanks for contributing to asdf!
+
+Your PR should trigger the CI (after approval for first-time contributors)
+which will:
+
+- check your code for 'style' using pre-commit
+- run your PR against the asdf unit tests for various OSes, python versions, and dependency versions
+- perform a test build of your PR
+
+It is highly recommended that you run some of these tests locally by:
+
+- [installing pre-commit](https://pre-commit.com/#quick-start)
+- [running pytest](https://docs.pytest.org/en/7.1.x/getting-started.html)
+
+This will increase the chances your PR will pass the required CI tests.
+-->
+
+# Description
+
+<!--
+Please describe what this PR accomplishes.
+If the changes are non-obvious, please explain how they work.
+If this PR adds a new feature please include tests and documentation.
+If this PR fixes an issue, please add closing keywords (eg 'fixes #XXX')
+-->
+
+# Checklist:
+
+- [ ] pre-commit checks ran successfully
+- [ ] tests ran successfully
+- [ ] for a public change, a changelog entry was added
+- [ ] for a public change, documentation was updated
+- [ ] for any new features, unit tests were added