Update issue templates

Author: fpistm

.github/ISSUE_TEMPLATE/feature-request-improvement.md

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+---
+name: Feature request/improvement
+about: Suggest an idea or improvement for this project
+title: ''
+labels: ''
+assignees: ''
+
+---
+
+**Is your feature request/improvement related to a problem? Please describe.**
+A clear and concise description of what the problem is. Ex. I'm always frustrated when [...]
+
+**Describe the solution you'd like**
+A clear and concise description of what you want to happen.
+
+**Describe alternatives you've considered**
+A clear and concise description of any alternative solutions or features you've considered.
+
+**Additional context**
+Add any other context, example or screenshots about the feature request here.
+
+Any contribution is welcome, so do not hesitate to submit a PR.

.github/ISSUE_TEMPLATE/new-variant-request.md

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+---
+name: New variant request
+about: Request for a new board support
+title: "[New Variant]"
+labels: New variant
+assignees: ''
+
+---
+
+**Board description (please complete the following information):**
+ - Name: [e.g. DISCOVERY F769NI]
+ - Link to all relevant information:
+   * Documentations
+   * Schematics
+   * Where this hardware can be purchased
+  * ...
+
+**Note that user can add a STM32 based board following this [wiki](https://github.com/stm32duino/wiki/wiki/Add-a-new-variant-(board))**
+
+Any contribution is welcome, so do not hesitate to submit a PR.