Developers: Add developers documentation

[syphax-bouazzouni]

Context

When we have new developers in the Alliance that don't just want to run the appliance but also want to do some development and enhancement to their local installation.

I think those people, will need at first urgently the following documentation pages:

• [x] How to set up a local development environment (done here https://github.com/ontoportal/documentation/pull/22)
• [x] Have an overview of how the code work (Global architecture) (done here https://github.com/ontoportal/documentation/pull/22)
• [ ] How to test their local code
• [ ] #24
• [ ] How to contribute to the Alliance