Documentation: Dev-environment instructions

[ZakarFin]

Instructions to spin up development environment with Java17

https://oskari.org/documentation/docs/unreleased/3-Setup-instructions has excellent setup instructions

So currently you just follow those instructions and make sure you have Java 17 installed, also make sure you have Java17 branch from Oskari github (oskari-server and sample-server-extension).

Additionally you can have oskari-frontend and sample-application running locally. Just change the sample-application package.json to reference the local frontend:

 "dependencies": {
    "oskari-frontend": "file:../oskari-frontend"
  }

and run npm install, then you can use npm run start:dev

[ZakarFin]

We need to update Jetty -> Tomcat at least after #85