Link `refinebio-examples/batch-effects` to RNA-seq module

@cbethell suggests we add more information about batch effects. I think the easiest way to do this is to link the the refine.bio-examples batch effect example after it is merged.

Check the refinebio-examples/batch-effects example and the RNA-seq module referenced in the issue, confirming that the example has been merged before changing the module. Add a link to that example in the relevant batch-effects information, then verify the link in the rendered module.

Written by the indexing model from the issue text.