Split out the fill layers into seperate pages, add multigrid docs.

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.