Add new workshop: Seurat DE

[vinisalazar]

Summary of changes:

• Add new workshop "scRNA-seq Integration and Differential Expression"
• Add images
• Add data files in txt

[vinisalazar]

Hi @manveerchauhan, I've formatted your workshop to follow the Material for MkDocs theme that we use for our website, as per @vperreau's request.

Have a look at commits 7c7458c, 05109cd and 2ebfd9c to see how it's done.

cc @ShiltonZhang to have a look as well.

Best, Vini