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:

• How to set up a local development environment (done here Feature: Add set up development environments docs #22)
• Have an overview of how the code work (Global architecture) (done here Feature: Add set up development environments docs #22)
• How to test their local code
• Dev/Devops: Write a documentation of how to deploy local code #24
• How to contribute to the Alliance