docs(angular): add @if content (#6371)

* docs(angular): add @if content

* Update src/data/roadmaps/angular/content/if@U1Zy2T-2ki9pDkXn9hn-I.md

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Co-authored-by: Kamran Ahmed <kamranahmed.se@gmail.com>

src/data/roadmaps/angular/content/if@U1Zy2T-2ki9pDkXn9hn-I.md

@@ -1 +1,8 @@
 # @if
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+The @if block conditionally displays its content when its condition expression is truthy. Content is added and removed from the DOM based on the evaluation of conditional expressions in the @if and @else blocks.
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+Visit the following resources to learn more:
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+- [@official@Angular Official Docs - @if](https://angular.dev/api/core/@if)
+- [@video@Narrow Down signal value type within an if statement](https://egghead.io/lessons/angular-narrow-down-angular-s-signal-value-type-within-an-if-statement)