As we're getting more and more fill layers, we should split them out into their own pages so they are easier to read and find.
We might need to reconsider this a little at some point, as internally what we're documenting is the generators, and the fill layer is just a filter layer but then for generators. This is especially important as @deiflou has made the new halftone layer use generators instead of only the screentone generator.
Anyhow, please read through for problems. I am currently still suffering from my wisdom tooth removal, AND typing this on the hottest day of the year till now. I also felt I had a hard time writing the multigrid page, so I am currently not sure if this MR is in actual English.
@lsegovia, we'll need to puzzle out how we're going to combine this branch and yours.