Add implementation details in [data/filetypes.restructuredtext](https://github.com/geany/geany/blob/master/data/filetypes.restructuredtext)
That file has been moved since then to [data/filedefs/filetypes.restructuredtext](https://github.com/geany/geany/blob/master/data/filedefs/filetypes.restructu...).
I'm working on this ticket atm.