Add issue and pull request templates (#2560)

.github/ISSUE_TEMPLATE/bug_report.md

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+---
+name: Bug report
+about: Report a bug that is preventing proper operation
+title: ''
+labels: Bugs
+assignees: ''
+
+---
+
+<!--
+If you are a user of OpenMC and are running into trouble with the code or are
+seeking general user support, we highly recommend posting on the OpenMC
+discourse forum first. GitHub issues should be used specifically for bug reports
+and feature requests.
+
+https://openmc.discourse.group/
+
+-->
+
+## Bug Description
+<!--A clear and concise description of the problem (Note: A missing feature is not a bug).-->
+
+
+## Steps to Reproduce
+<!--Steps to reproduce the behavior (input file, or modifications to an existing input file, etc.)-->
+
+
+## Environment
+<!--Operating system, OpenMC version, how OpenMC was installed, nuclear data being used, etc.-->

.github/ISSUE_TEMPLATE/config.yml

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+blank_issues_enabled: false
+contact_links:
+  - name: Troubleshooting and User Support
+    url: https://openmc.discourse.group/
+    about: For user support and troubleshooting, please use our Discourse forum

.github/ISSUE_TEMPLATE/documentation.md

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+---
+name: Documentation improvement
+about: Found something incomplete or incorrect in our documentation?
+title: ''
+labels: Documentation
+assignees: ''
+
+---
+
+<!--Describe the issue with the documentation and include a URL to the corresponding page-->

.github/ISSUE_TEMPLATE/feature_request.md

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+---
+name: Feature request
+about: Suggest a new feature or enhancement to existing capabilities
+title: ''
+labels: ''
+assignees: ''
+
+---
+
+## Description
+<!--What is the feature or enhancement?-->
+
+
+## Alternatives
+<!--If alternative solutions have been considered, describe them here and the reasoning for the chosen solution --->
+
+
+## Compatibility
+<!--Will the enhancement change existing APIs or add something new?-->

.github/pull_request_template.md

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+<!--
+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
+-->