chore(github): add issue templates (#750)

relates to STACKITTPR-138

Author: rubenhoenle

.github/ISSUE_TEMPLATE/bug_report.md

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+---
+name: Bug report
+about: Report a bug in the STACKIT Terraform provider
+title: ''
+labels: bug
+assignees: ''
+
+---
+
+## Description
+
+*Please add a clear and concise description of what the bug is.*
+
+## Steps to reproduce
+<!-- Please add an example terraform config below which helps us reproduce the behavior. -->
+
+```terraform
+# put your terraform example here
+# ...
+```
+
+<!-- Please provide us with the steps to reproduce the behavior. -->
+
+1. Run `terraform ...`
+2. ...
+
+## Actual behavior
+
+*Please describe the current behavior of the STACKIT Terraform provider. Don't forget to add detailed information like error messages.*
+
+## Expected behavior
+
+*Please describe the behavior which you would expect from the STACKIT Terraform provider in that case.*
+
+## Environment
+ - OS: 
+ - Terraform version (see `terraform --version`): `vX.X.X` 
+ - Version of the STACKIT Terraform provider: `vX.X.X`
+
+**Additional information**
+
+*Feel free to add any additional information here.*

.github/ISSUE_TEMPLATE/feature_request.md

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+---
+name: Feature request
+about: Suggest an idea for the STACKIT Terraform provider
+title: ''
+labels: enhancement
+assignees: ''
+
+---
+
+## Problem description
+
+*Is your feature request related to a problem? If so, please give us a clear and concise description of what the problem is. 
+Example: I'm always frustrated when [...]*
+
+## Proposed solution
+
+*A clear and concise description of what you want to happen.*
+
+<!-- Please add an example terraform config below which helps us understand your proposed solution. -->
+
+```terraform
+# put your terraform example here
+# ...
+```
+
+## Alternative solutions (optional)
+
+*A clear and concise description of any alternative solutions or features you've considered. (optional)*
+
+## Additional information
+
+*Feel free to add any additional information here.*
+