Link dada2 and qiime2 tutorials

[bernt-matthias]

• dada2: https://benjjneb.github.io/dada2/tutorial.html The tutorial is for R, but since the Galaxy tools wrap the R functions 1:1 it should be easy to follow.
• qiime2: https://docs.qiime2.org/jupyterbooks/cancer-microbiome-intervention-tutorial/index.html (for the steps involving computation users can select if the tutorial should be shown for Galaxy, CLI, or python, e.g. here)

workflows will be soon(ish) at IWC, e.g. dada2 here https://github.com/galaxyproject/iwc/pull/14

[bebatut]

For QIIME2 tutorial: https://github.com/galaxyproject/training-material/pull/4715