Developer doc restructuring

This PR is a first attempt to restructure the Bio-Formats developer documentation in order to ease its usage and its extension by developers. The index is divided into 4 major sections 

1- introduction (getting/building the code, introduction to the API)
2- Bio-Formats / Java
3- Bio-Formats via C++
4- contributing

/cc @hflynn @melissalinkert 

I know this was Josh's suggestion but I don't think this title makes the content clear. 
Writing a new file format reader?
Adding support for a new file format?