Add 0_on_premises

Author: renke

app/.mvn/wrapper/maven-wrapper.jar

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+distributionUrl=https://repo.maven.apache.org/maven2/org/apache/maven/apache-maven/3.9.2/apache-maven-3.9.2-bin.zip
+wrapperUrl=https://repo.maven.apache.org/maven2/org/apache/maven/wrapper/maven-wrapper/3.2.0/maven-wrapper-3.2.0.jar

app/.mvn/wrapper/maven-wrapper.properties

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+# 0 – On-Premises
+
+In this exercise we want to set up everything we need for the other exercises. Also we want to take a look at the example application (aka the OK Forum).
+
+Note: Make sure you current directory is now `0_on_premises`.
+
+1. Go to the `frontend` directory
+
+    - Run `npm install` and `npm run dev`
+    - A message should pop up which tells how to to reach the frontend
+
+2. Now go back to `0_on_premises` directory in a new terminal
+
+    - Run `sdk install java 17.0.0-tem` (and possibly `sdk default java 17.0.0-tem` and `sdk use java 17.0.0-tem`)
+    - Check if you are using Java 17 by running `java -version`
+    - Run `mvn spring-boot:run` to start the backend
+    - A message should pop up which tells us how to reach the backend
+    - Make the backend port public (this can be done in the port tab via right click menu)
+
+3. Connect the frontend to backend
+
+    - Adjust the showcase "0 – On-Premises" in showcases.ts
+    - Set the base URL to the URL of the backend
+    - Select showcase "0 – On-Premises" and check if the app works properly
+
+4. Stop the backend by pressing Ctrl+C in the backend terminal