README: Add cheat sheets, newsletter, mastering data structures

Author: SvenWoltmann

README.md

@@ -30,15 +30,15 @@ curl localhost:8080/stage1-seq/product/1
 Within IntelliJ, you can also open the `endpoints-test.http` file to run the HTTP requests.
 
 
-## Java 21 IDE Support
+## <br>Java 21 IDE Support
 
 [IntelliJ IDEA](https://www.jetbrains.com/idea/) supports Java 21 as of version 2023.2.2.
 As long as this version is not yet released,
 you can install the release candidate via the [JetBrains Toolbox App](https://www.jetbrains.com/toolbox-app/)
 by going into the IntelliJ IDEA settings and activating the "Early Access Program".
 
 
-## Java Downloads
+## <br>Java Downloads
 
 You can download Java 21 from here: https://jdk.java.net/21/
 
@@ -48,7 +48,7 @@ To install multiple Java versions on Windows,
 have a look at this tutorial: [How to Change Java Versions in Windows](https://www.happycoders.eu/java/how-to-switch-multiple-java-versions-windows/)
 
 
-## Other Java 21 Examples
+## <br>Other Java 21 Examples
 
 You might also find these GitHub repositories interesting:
 
@@ -65,3 +65,51 @@ Scoped Values:
 Pattern Matching for Switch:
 * https://github.com/SvenWoltmann/pattern-matching-for-switch
 
+
+## <br>Other Resources
+
+### Java Versions PDF Cheat Sheet
+
+**Stay up-to-date** with the latest Java features with [this PDF Cheat Sheet](https://www.happycoders.eu/java-versions/)!
+
+[<img src="/img/Java_Versions_PDF_Cheat_Sheet_Mockup_936.v2.png" alt="Java Versions PDF Cheat Sheet Mockup" style="width: 468px; max-width: 100%;">](https://www.happycoders.eu/java-versions/)
+
+* Avoid lengthy research with this **concise overview of all Java versions up to Java 23**.
+* **Discover the innovative features** of each new Java version, summarized on a single page.
+* **Impress your team** with your up-to-date knowledge of the latest Java version.
+
+👉 [Download the Java Versions PDF](https://www.happycoders.eu/java-versions/)<br>
+
+_(Hier geht's zur deutschen Version &rarr; [Java-Versionen PDF](https://www.happycoders.eu/de/java-versionen/))_
+
+
+### <br>The Big O Cheat Sheet
+
+With this [1-page PDF cheat sheet](https://www.happycoders.eu/big-o-cheat-sheet/), you'll always have the **7 most important complexity classes** at a glance.
+
+[<img src="/img/big-o-cheat-sheet-pdf-en-transp_936.v2.png" alt="Big O PDF Cheat Sheet Mockup" style="width: 468px; max-width: 100%;">](https://www.happycoders.eu/big-o-cheat-sheet/)
+
+* **Always choose the most efficient data structures** and thus increase the performance of your applications.
+* **Be prepared for technical interviews** and confidently present your algorithm knowledge.
+* **Become a sought-after problem solver** and be known for systematically tackling complex problems.
+
+👉 [Download the Big O Cheat Sheet](https://www.happycoders.eu/big-o-cheat-sheet/)<br>
+
+_(Hier geht's zur deutschen Version &rarr; [O-Notation Cheat Sheet](https://www.happycoders.eu/de/o-notation-cheat-sheet/))_
+
+
+### <br>HappyCoders Newsletter
+👉 Want to level up your Java skills?
+Sign up for the [HappyCoders newsletter](http://www.happycoders.eu/newsletter/) and get regular tips on programming, algorithms, and data structures!
+
+_(Hier geht's zur deutschen Version &rarr; [HappyCoders-Newsletter deutsch](https://www.happycoders.eu/de/newsletter/))_
+
+
+### <br>🇩🇪 An alle Java-Programmierer, die durch fundierte Kenntnisse über Datenstrukturen besseren Code schreiben wollen
+
+Trage dich jetzt auf die [Warteliste](https://www.happycoders.eu/de/mastering-data-structures-warteliste/) von „Mastering Data Structures in Java“ ein, und erhalte das beste Angebot!
+
+[<img src="/img/mastering-data-structures-product-mockup-cropped-1600.v2.png" alt="Mastering Data Structures Mockup" style="width: 640px; max-width: 100%;">](https://www.happycoders.eu/de/mastering-data-structures-warteliste/)
+
+👉 [Zur Warteliste](https://www.happycoders.eu/de/mastering-data-structures-warteliste/)
+