Add issue templates

.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: ''
+
+---
+
+## 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/documentation-improvement.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?-->