Documentation of dither is missing

[JosepMaJAZ]

The documentation of the dither option is missing.

In the documentation there's a dither.html file which is just an outdated copy of the command.html file with a comment that the dither option needs to be added.

The only "documentation" in existence is the dither_test.c file.

[erikd]

The dithering code was never completed and hence never documented.

[erikd]

I've deleted the file doc/dither.html from the repo. It can be resurrected when and if dither is actually implemented.