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

/cc @manics @mtbc @rleigh-dundee @hflynn @kennethgillen