"Contribuing to OMERO" developer doc rework

* Rename files to follow abc-xyz.txt standard
 * Reduce the number of individual files
 * Improve the order of files for external developers
 * Improve the order of sections in some files
 * Drop UsingGitFlow

Looking at developers/index.txt and developers/contributing.txt
it's still not completely clear 1) that a developer should REALLY
read the "contributing" section, nor 2) once they've opened up
contributing.txt which sections are highly important.

NB: very little has been done in the way of optimizing the PDF