Add guide for Java example app

[tschale]

We should create a guide for our java spring example (https://github.com/blueshoe/java-spring-example and https://github.com/blueshoe/java-spring-example-charts).

I got to the point where I was able to run the image in a k3d cluster, with following commands:

docker build --target dev -t java-spring:dev .
gefyra up
gefyra run -i java-spring:dev -N java-demo -v /home/gutschi/Projects/blog/java-spring-example/polls:/app -n default --env JAVA_HOME=/usr/local/openjdk-17 --env-from deploy/polls --expose 8080:8080

JAVA_HOME needs to be specified, as the base-image in the development stage uses a different one than the base-image in the production stage...

Actual hot reloading didn't work, Veit issued the assumption that it may work in a setup with IntelliJ, where there's maven support configured. I couldn't get it to work in a short time, so I stopped there.

ToDos:

• finish run with hot reload (or confirm that it won't work)
• gefyra bridge
• write guide