GENtle package updated - dropped GFDL licensed documentation

Dear Étienne,

thank you tons for pointing out that the license of the documentation is not as Free as it should be. I guess this will be addressed with the next major release. For now I have removed docs/manual.adoc and docs/images from the source tree as you suggested.

https://salsa.debian.org/med-team/gentle

I am testing this mostly on a Mac and just ran into some platform-specific bug, but, anyway this version is better than the currently in the archive and so this should be uploaded :)

Many thanks,
Steffen