add issue templates (#404)

.github/ISSUE_TEMPLATE/bug_report.yaml

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+name: Bug Report
+description: Create a report if something doesn't work quite right.
+labels: ["bug", "needs triage"]
+
+body:
+- type: markdown
+  attributes:
+    value: |
+      Thanks for taking the time to fill out this bug report!
+
+- type: checkboxes
+  id: checks
+  attributes:
+    label: Version Checks (indicate both or one)
+    options:
+    - label: >
+        I have confirmed this bug exists on the lastest
+        [release](https://github.com/pypsa/linopy/releases) of Linopy.
+    - label: >
+        I have confirmed this bug exists on the current
+        [`master`](https://github.com/pypsa/linopy/tree/master) branch of Linopy.
+
+- type: textarea
+  id: problem
+  attributes:
+    label: Issue Description
+    description: >
+      Please provide a description of the issue.
+  validations:
+    required: true
+
+- type: textarea
+  id: example
+  validations:
+    required: true
+  attributes:
+    label: Reproducible Example
+    description: >
+      Please provide a minimal reproduciable example. See how to [craft minimal bug reports](https://matthewrocklin.com/minimal-bug-reports).
+    placeholder: >
+      from linopy import Model
+
+      m = Model()
+
+    render: python
+
+- type: textarea
+  id: expected-behavior
+  validations:
+    required: true
+  attributes:
+    label: Expected Behavior
+    description: >
+      Please describe or show a code example of the expected behavior.
+
+- type: textarea
+  id: version
+  attributes:
+    label: Installed Versions
+    description: >
+      Please share information on your environment. Paste the output below.  For conda ``conda env export`` and for pip ``pip freeze``.
+    value: >
+      <details>
+
+      Replace this line.
+
+      </details>

.github/ISSUE_TEMPLATE/config.yml

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+blank_issues_enabled: true

.github/ISSUE_TEMPLATE/feature_request.md

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+---
+name: Feature Request
+about: Suggest an idea for this project
+title: ''
+labels: ["feature", "needs triage"]
+assignees: ''
+
+---
+
+## Describe the feature you'd like to see
+
+*Please give a clear and concise description and provide context why the feature would be useful.*
+*Also, we'd appreciate any implementation ideas and references you already have.*

.github/pull_request_template.md

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+Closes # (if applicable).
+
+## Changes proposed in this Pull Request
+
+
+## Checklist
+
+- [ ] Code changes are sufficiently documented; i.e. new functions contain docstrings and further explanations may be given in `doc`.
+- [ ] Unit tests for new features were added (if applicable).
+- [ ] A note for the release notes `doc/release_notes.rst` of the upcoming release is included.
+- [ ] I consent to the release of this PR's code under the MIT license.