feat: add doc

docs/src/content/docs/challenges/angular/43-signal-input.md

@@ -1,6 +1,6 @@
 ---
 title: 🟢 Signal Input
-description: Challenge 43 is about ...
+description: Challenge 43 is about learning how to use signal inputs
 author: thomas-laforge
 challengeNumber: 43
 command: angular-signal-input
@@ -9,12 +9,44 @@ sidebar:
   badge: New
 ---
 
-:::note
-WIP: The following documentation need to be written.
-:::
-
 ## Information
 
+Finally, the day has arrived when the Angular team introduces a reactive input. This highly requested feature has been awaited for many years. Version 17.1 introduces `SignalInput`. Instead of utilizing the well-known `@Input` decorator, you now have a function that returns a signal.
+
+```ts
+// old way
+@Input() age?: number;
+
+// new way
+age = input<number>()
+```
+
+If you want required inputs
+
+```ts
+// old way
+@Input({required: true}) age!: number;
+
+// new way
+age = input.required<number>()
+```
+
+If you wanted to obtain a signal from an input, you had to go through a setter to configure your signal from the input.
+
+```ts
+// old way
+age = signal(0)
+@Input({alias: 'age'}) set _age(age: number){
+  this.age.set(age)
+};
+
+// new way
+age = input<number>()
+```
+
 ## Statement
 
-## Constraints
+In this small application, the goal is to refactor the `UserComponent` to utilize `SignalInput`.
+
+- You have required and optional inputs.
+- You can use the `transform` function for the `age` input to directly convert the property to a number.