OMERO developer setup 

This PR is meant to address https://trello.com/c/sb3mPUwV/190-developer-setup-docs-walkthrough and various reports about missing information for developers.

Major changes are the following:
- all the advanced Git configuration is stripped from the developer documentation and now should live exclusively on the contributing documentation
- a link to the contributing documentation is added for people wanting to contribute
- a placeholder is added to list the OMERO build dependencies (libraries, envvars)
- the main build commands are listed on the page a link to the advanced build system page

Additional points to discuss:
- which additional build libraries/envvars would be required?
- should we add an alternate way of getting the source code using the OMERO source code zip from the downloads page? Note the latter does not include the Bio-Formats versioning yet.
- should `Installing OMERO` be renamed as `Building OMERO` ?

/cc @manics @mtbc @rleigh-dundee @hflynn @kennethgillen 
While there's nothing factually wrong with the CentOS/Ubuntu examples here, I think this section could benefit from being split like the unix install instructions where we explicitly state what the general dependencies are, and then provide the specifics for all platforms.  This, for example, neglects the MacOS dependencies.  This PR is already quite complex, so this could be deferred for a followup PR.