OMERO developer setup  (rebased onto dev_5_0)


This is the same as gh-954 but rebased onto dev_5_0.

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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 

                
Ok, thanks!