users-guide: Disable index node generation
users-guide: Disable index node generation

This is breaking the build on some platforms. It's unclear exactly why but I
don't have time to investigate at the moment.

shlevy/ghc,ezyang/ghc,sdiehl/ghc,shlevy/ghc,ezyang/ghc,sdiehl/ghc,shlevy/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,shlevy/ghc,ezyang/ghc,sdiehl/ghc,shlevy/ghc,ezyang/ghc,shlevy/ghc,shlevy/ghc,ezyang/ghc,sdiehl/ghc,ezyang/ghc,ezyang/ghc