Provide setup instructions for developers to get Mauro configured end-to-end

[pjmonks]

The various MauroDataMapper repos all have README files which start to explain how to setup each repo, but with no clear instructions for a developer to setup a full development environment. One example is for a developer to work on mdm-ui - this also requires setup of a development Mauro backend, but these instructions are not obvious from the mdm-ui repo.

The suggestion from @olliefreeman is to avoid duplicating possible steps in multiple README files and instead consolidate these setup instructions with the docs repo, provided under a "developers" section. All the repos would then be stripped down and point to this single source of truth, which will also help maintainability.

So minimal steps should cover:

1. Dependencies required for running mdm-core - Postgresql, Groovy, particular versions, etc
2. How to start mdm-core
3. How to start mdm-application-build and use cases for it
4. How to run mdm-ui and how to prepare a backend for testing