Workflow chapter using Xenium dataset

[lmweber]

Following on from discussion in #42 @estellad

In this issue we are adding a workflow chapter using the Xenium dataset adapted from @estellad's BC2 workflow materials. This will also be a great addition, since we currently do not have any examples with a Xenium dataset.

As discussed in #42 , we prefer to avoid using Seurat objects, so we could try using MoleculeExperiment instead to load the data. The MoleculeExperiment package includes a data loading function readXenium(). I haven't worked with MoleculeExperiment much yet myself though so please let me know how this goes. If this works, we could also add the MoleculeExperiment object to STexampleData (or a separate similar package) to streamline loading in the examples.

Also tagging @stephaniehicks @raphg