add issue templates

Author: MalteKurz

.github/ISSUE_TEMPLATE/api_docu.yml

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+name: API Documentation Improvement
+description: Suggest an improvement for the API documentation.
+title: "[API Documentation]: "
+labels: ["documentation"]
+
+body:
+- type: textarea
+  attributes:
+    label: Describe the issue related to the API documentation
+  validations:
+    required: true
+- type: textarea
+  attributes:
+    label: Suggested alternative or fix

.github/ISSUE_TEMPLATE/bug_report.yml

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+name: Bug Report
+description: File a bug report
+title: "[Bug]: "
+labels: ["bug"]
+assignees:
+  - PhilippBach
+
+body:
+  - type: markdown
+    attributes:
+      value: |
+        Thanks for taking the time to fill out this bug report!
+  - type: textarea
+    id: description
+    attributes:
+      label: Describe the bug
+      description: |
+        Please provide a clear and concise discription of the bug.
+    validations:
+      required: true
+  - type: textarea
+    id: mwe
+    attributes:
+      label: Minimum reproducible code snippet
+      description: |
+        Please provide a short reproducible code snippet. Example:
+
+        ```R
+        library(DoubleML)
+        library(mlr3)
+        library(mlr3learners)
+        library(data.table)
+        set.seed(2)
+        ml_g = lrn("regr.ranger", num.trees = 10, max.depth = 2)
+        ml_m = ml_g$clone()
+        obj_dml_data = make_plr_CCDDHNR2018(alpha = 0.5)
+        dml_plr_obj = DoubleMLPLR$new(obj_dml_data, ml_g, ml_m)
+        dml_plr_obj$fit()
+        dml_plr_obj$summary()
+        ```
+      placeholder: |
+
+        ```R
+        Sample code here
+        ```
+    validations:
+      required: true
+  - type: textarea
+    id: expected
+    attributes:
+      label: Expected Result
+      description: |
+        State the result you would have expected
+    validations:
+      required: true
+  - type: textarea
+    id: actual
+    attributes:
+      label: Actual Result
+      description: |
+        State the result you get (for example an exception including the full traceback)
+    validations:
+      required: true
+  - type: textarea
+    id: versions
+    attributes:
+      label: Versions
+      description: |
+        Please run the following code and copy-paste the result
+
+        ```R
+        sessionInfo()
+        packageVersion('DoubleML')
+        packageVersion('mlr3')
+        ```
+    validations:
+      required: true

.github/ISSUE_TEMPLATE/config.yml

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+blank_issues_enabled: true
+contact_links:
+  - name: Discussions
+    url: https://github.com/DoubleML/doubleml-for-r/discussions/new
+    about: Ask questions about the DoubleML package or start related dicussions
+  - name: Documentation and User Guide
+    url: https://github.com/DoubleML/doubleml-docs/issues/new/choose
+    about: Issues related to the documentation and user guide on docs.doubleml.org should be reported in the doubleml-docs repo
+  - name: Blank Issue
+    url: https://github.com/DoubleML/doubleml-for-r/issues/new
+    about: Please note that often Dicussions is the better choice

.github/ISSUE_TEMPLATE/feature_request.yml

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+name: Feature Request
+description: Suggest an enhancement or extension for the DoubleML package
+title: "[Feature Request]: "
+labels: ["new feature", "enhancement"]
+
+body:
+- type: textarea
+  attributes:
+    label: Describe the feature you want to propose or implement
+  validations:
+    required: true
+- type: textarea
+  attributes:
+    label: Propose a possible solution or implementation
+- type: textarea
+  attributes:
+    label: Did you consider alternatives to the proposed solution. If yes, please describe
+- type: textarea
+  attributes:
+    label: Comments, context or references