fixup! Add AI, index and review subject for Tron

subjects/abs.en.md

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+## Abs
+
+### Instructions
+
+Create a `isPositive` function that takes a number as
+parameter and return true if the given number is
+stricly positive, or false otherwise
+
+Create the `abs` function that takes one number argument
+and returns its absolute value.
+You are not allowed to use `Math.abs`, make your own.
+
+
+### Notions
+
+- [nan-academy.github.io/js-training/examples/functions.js](https://nan-academy.github.io/js-training/examples/functions.js)
+- [nan-academy.github.io/js-training/examples/ternary.js](https://nan-academy.github.io/js-training/examples/ternary.js)
+- [devdocs.io/javascript/global_objects/math/abs](https://devdocs.io/javascript/global_objects/math/abs)
+
+
+### Code provided
+
+> all code provided will be added to your solution and doesn't need to be submited.
+
+```js
+Math.abs = undefined
+```