Add CellRank to Scanpy's ecosystem

[michalk8]

Hi,
this PR addresses #1282 and ads CellRank to the ecosystem page.
As for the other 2 packages, @Marius1311 and I've decided here theislab/cellrank#241 that we will postpone adding then 'till we're satisfied with them.
Let me know if there's anything I need to add w.r.t. to this PR.

[ivirshup]

It might be useful to have a sentence describing the package, but isn't necessary. Are you interested in adding that?

[Marius1311]

Sure! @michalk8 , could you please add the below sentence:

"CellRank is a toolkit to uncover cellular dynamics based on scRNA-seq data with RNA velocity annotation by detecting initial and terminal populations, inferring fate potentials and uncovering gene expression trends towards specific terminal populations. "

[Marius1311]

Thanks @michalk8 !

[Marius1311]

Thanks @ivirshup !