docs(angular): add elf content (#6499)

src/data/roadmaps/angular/content/elf@rgPUcSKxG9DvXicLfC2Ay.md

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 # Elf
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+Elf is a reactive immutable state management solution built on top of RxJS. It uses custom RxJS operators to query the state and pure functions to update it. Elf encourages simplicity. It saves you the hassle of creating boilerplate code and offers powerful tools with a moderate learning curve, suitable for experienced and inexperienced developers alike.
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+Visit the following resources to learn more:
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+- [@official@What is Elf?](https://ngneat.github.io/elf/)
+- [@opensource@Elf](https://github.com/ngneat/elf)
+- [@opensource@Elf NG Router Store](https://github.com/ngneat/elf-ng-router-store)
+- [@article@Case Study: Elf](https://medium.com/@gronichomer/case-study-elf-%EF%B8%8F%EF%B8%8F-part-1-fe5e87c31c89)