added issue & PR templates

Signed-off-by: Jenn Nguyen <jenn@openrobotics.org>

.github/ISSUE_TEMPLATE/bug_report.md

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+---
+name: Bug report
+about: Report a bug
+labels: bug
+---
+
+<!-- If you're not sure whether your problem is a bug, please ask a question at
+http://answers.gazebosim.org instead.-->
+
+## Environment
+* OS Version: <!-- e.g, Ubuntu 20.04 -->
+* Source or binary build?
+  <!-- If binary, which version? -->
+  <!-- If source, which branch and what is the latest commit? -->
+
+## Description
+* Expected behavior: <!-- Tell us what you expected to happen -->
+* Actual behavior: <!-- What happened instead -->
+
+## Steps to reproduce
+<!-- Provide steps so we can try to reproduce this issue -->
+
+1.
+2.
+3.
+
+## Output
+<!-- Provide screenshots, console logs, backtraces, and/or anything that could
+be useful to us resolving this issue -->

.github/ISSUE_TEMPLATE/feature_request.md

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+---
+name: Feature request
+about: Request a new feature
+labels: enhancement
+---
+
+<!-- If you're not sure on the specifics of the feature or would like a broader
+discussion, please ask consider posting a proposal to
+http://community.gazebosim.org/ instead.-->
+
+## Desired behavior
+<!-- Describe the current problem and the feature you want implemented.-->
+
+## Alternatives considered
+<!-- Describe alternate solutions or features you've considered.-->
+
+## Implementation suggestion
+<!-- Provide a suggestion on how to implement this feature, which could help us
+expedite this implementation.-->
+
+## Additional context
+<!-- Provide any other additional context (e.g., screenshots) about your feature
+request.-->

.github/PULL_REQUEST_TEMPLATE.md

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+<!-- Please remove the appropriate section.
+For example, if this is a new feature, remove the "Bug Report" section -->
+
+# Bug Report
+
+Fixes issue #<NUMBER>
+
+## Summary
+<!-- Describe your fix, including an explanation of how to reproduce the bug
+before and after the PR.-->
+
+## Checklist
+- [ ] Signed all commits for DCO
+- [ ] Added tests
+- [ ] Updated documentation (as needed)
+- [ ] Updated migration guide (as needed)
+- [ ] `codecheck` passed (See
+  [contributing](https://ignitionrobotics.org/docs/all/contributing#contributing-code))
+- [ ] All tests passed (See
+  [test coverage](https://ignitionrobotics.org/docs/all/contributing#test-coverage))
+- [ ] While waiting for a review on your PR, please help review
+[another open pull request](https://github.com/pulls?q=is%3Aopen+is%3Apr+user%3Aignitionrobotics+archived%3Afalse+)
+to support the maintainers
+
+**Note to maintainers**: Remember to use **Squash-Merge**
+
+---
+
+# New feature
+
+Closes #<NUMBER>
+
+## Summary
+<!--Explain changes made, the expected behavior, and provide any other additional
+context (e.g., screenshots, gifs) if appropriate.-->
+
+## Test it
+<!--Explain how reviewers can test this new feature manually.-->
+
+## Checklist
+- [ ] Signed all commits for DCO
+- [ ] Added tests
+- [ ] Added example world and/or tutorial
+- [ ] Updated documentation (as needed)
+- [ ] Updated migration guide (as needed)
+- [ ] `codecheck` passed (See [contributing](https://ignitionrobotics.org/docs/all/contributing#contributing-code))
+- [ ] All tests passed (See [test coverage](https://ignitionrobotics.org/docs/all/contributing#test-coverage))
+- [ ] While waiting for a review on your PR, please help review
+[another open pull request](https://github.com/pulls?q=is%3Aopen+is%3Apr+user%3Aignitionrobotics+archived%3Afalse+)
+to support the maintainers
+
+**Note to maintainers**: Remember to use **Squash-Merge**

.github/PULL_REQUEST_TEMPLATE/port.md

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+Port <FROM_BRANCH> to <TO_BRANCH>
+
+Branch comparison: https://github.com/ignitionrobotics/ign-gui/compare/<FROM_BRANCH>...<TO_BRANCH>
+
+**Note to maintainers**: Remember to **Merge** with commit (not squash-merge
+or rebase)