move mothur tutorials to the end

[bernt-matthias]

• mothur is super outdated, seems not updated anymore and creates lots of problems
• unfurtunatelly we have an excellent tutorial and people keep using it
• so lets at least put it on the end of the list

Was also wondering if we could/should add more structure to this topic. In particular I think it might be good to differentiate between amplicon and (real) metagenomics. Maybe by subtopics or just tags?

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[shiltemann]

If/when we have a suitable replacement tutorial, we can also add a note to the top of the tutorial saying that we recommend this other tutorial