Documentation for building JDT

[stephan-herrmann]

Currently, when you look for information how to build JDT, you're lost in an infinite reference loop:

From https://github.com/eclipse-jdt/.github/blob/main/CONTRIBUTING.md:

Information regarding source code management, builds, coding standards, and more.

• https://projects.eclipse.org/projects/eclipse.jdt/developer

That latter page simply links back to github.

For normal development, having a working Eclipse workspace (preferably via Oomph) is of course sufficient, but should you ever need to build with maven, it's hard to find a command line that actually works (plus all the prerequisites like maven version, toolchains.xml etc. pp.).