outsource demo sections where appropriate

[joey711]

Some demo sections have more-updated tutorials on the phyloseq main page tutorials. These sections should simply link to those and show nothing, with the exception of material that is new, but too long, involved, specific, or otherwise not appropriate to migrate to the main page tutorials. This will save time rebuilding demo pages, keep the connection between these two documentation sites robust, and make the management and maintenance of demo code (both demo and main page), much easier according to the DRY principle.