Split backends into multiple repositories

[mojavelinux]

In order to distribute the backends, I think it will work best if we split them into multiple repositories.

• There should be one repository to hold the clones of the built-in backends (html5, docbook45 and manpage)
• Each slideshow framework should have its own repository since they tend to move at a different pace
• There should be one repository where new backends go, an incubator so to speak

[lordofthejars]

👍 to use a repo as incubator. And it is a good idea to have one repo for each backend. Moreover because currently it is been used by lot of people, so it would be easy to open issues exactly on the backend where the error occurs. Moreover we also could do some kind of statistics to know which are the most used, so we can focus on them.