Separate and update coding conventions and contributing pages

doc/coding-convention.md

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+# Coding convention
+
+## Language
+
+The language of this project is **C++**, and all new code must be written in C++. (Modern) C++ provides a lot of useful tools and functionalities that are beneficial for embedded software development like `constexpr`, `template` and anything that provides zero-cost abstraction.
+
+C code is accepted if it comes from another library like FreeRTOS, NimBLE, LVGL or the NRF-SDK.
+
+## Coding style
+
+The most important rule to follow is to try to keep the code as easy to read and maintain as possible.
+
+Using an autoformatter is highly recommended, but make sure it's configured properly.
+
+There are preconfigured autoformatter rules for:
+
+  * CLion (IntelliJ) in [.idea/codeStyles/Project.xml](/.idea/codeStyles/Project.xml)
+  * `clang-format`
+
+Also use `clang-tidy` to check the code for other issues.
+
+If there are no preconfigured rules for your IDE, you can use one of the existing ones to configure your IDE.
+
+ - **Indentation** : 2 spaces, no tabulation
+ - **Opening brace** at the end of the line
+ - **Naming** : Choose self-describing variable name
+    - **class** : PascalCase
+    - **namespace** : PascalCase
+    - **variable** : camelCase, **no** prefix/suffix ('_', 'm_',...) for class members
+ - **Include guard** : `#pragma once` (no `#ifdef __MODULE__ / #define __MODULE__ / #endif`)
+ - **Includes** :
+    - files from the project : `#include "relative/path/to/the/file.h"`
+    - external files and std : `#include <file.h>`
+ - Only use [primary spellings for operators and tokens](https://en.cppreference.com/w/cpp/language/operator_alternative)
+ - Use auto sparingly. Don't use auto for [fundamental/built-in types](https://en.cppreference.com/w/cpp/language/types) and [fixed width integer types](https://en.cppreference.com/w/cpp/types/integer), except when initializing with a cast to avoid duplicating the type name.
+ - Examples:
+   - `auto* app = static_cast<DisplayApp*>(instance);`
+   - `auto number = static_cast<uint8_t>(variable);`
+   - `uint8_t returnValue = MyFunction();`
+ - Use nullptr instead of NULL