doc/Makefile: drop doc-diff worktree and temporary files on "make clean"

doc-diff creates a temporary working tree (git-worktree) and generates a bunch of temporary files which it does not remove since they act as a cache to speed up subsequent runs. Although doc-diff's working tree and generated files are not strictly build products of the Makefile (which, itself, never runs doc-diff), as a convenience, update "make clean" to clean up doc-diff's working tree and generated files along with other development detritus normally removed by "make clean". Signed-off-by: Eric Sunshine <sunshine@sunshineco.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>