docs(angular): add structural directive content (#6524)

src/data/roadmaps/angular/content/structural-directives@xk3v8p6vf8ntGj5c-IU4U.md

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 # Structural Directives
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+Structural directives are directives applied to an `<ng-template>` element that conditionally or repeatedly renders the content of that `<ng-template>`. If you just wrap elements in an `<ng-template>` without applying a structural directive, those elements will not be rendered.
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+In Angular, there are three standard structural directives:
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+- `*ngIf` – conditionally includes a template depending on the value of an expression returned by a Boolean.
+- `*ngFor` – makes it simple to iterate over an array.
+- `*ngSwitch` – renders each matching view.
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+Visit the following resources to learn more:
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+- [@official@Angular Official Docs - Structural Directives](https://angular.dev/guide/directives/structural-directives)
+- [@article@Structural Directives in Angular](https://medium.com/@eugeniyoz/structural-directives-in-angular-61fe522f3427)
+- [@article@Angular Structural Directive Patterns: What they are and how to use them](https://www.freecodecamp.org/news/angular-structural-directive-patterns-what-they-are-and-how-to-use-them/)