Merge branch 'jk/diff-rendered-docs'

Dev doc update. * jk/diff-rendered-docs: Revert "doc/Makefile: drop doc-diff worktree and temporary files on "make clean"" doc/Makefile: drop doc-diff worktree and temporary files on "make clean" doc-diff: add --clean mode to remove temporary working gunk doc-diff: fix non-portable 'man' invocation doc-diff: always use oids inside worktree SubmittingPatches: mention doc-diff