doc: Fix compilation problems of the docs target on non-Linux machines (tested on FreeBSD)

On non-linux machines Zotonic is compiled with `gmake`, not `make`, thus the makefile has to use the $(MAKE) variable to correctly invoke the correct make executable.

Also, on non-linux machines `bash` isn't in `/bin/bash`. I've changed it so that it is compatible with linux and non-linux machines.

Finally `rm` reported errors if the file to delete didn't exist (adding `-f` removes that error)

Also adding one `.rst` file which was probably missing because it was created automatically when I tried to compile `docs`.