nbconvert: Serve from original build directory

The new `serve` post processor for nbconvert has a `build_directory` argument which I think is not really useful. I cannot really think of a situation where you want it to be different than the build directory of the original converter.

This change loses the `build_directory` argument and uses the path of the input file.

However, I'm not sure this plays well with notebooks in subdirectories. It certainly doesn't work well with external files referenced by relative paths, but I also don't think the original approach did. So it might need some more thinking.