Improve data access organization and documentation

[lcolladotor]

Currently available datasets are specified as arguments to the fetch_data() function. The documentation can be improved and the available datasets can be reorganized. We need to re-design this and create a vignette documenting available datasets, list related publications, include abstracts of the available datasets, link to the project GitHub repositories, among other enhancements.

[lahuuki]

Details to add about spatialDLPFC snRNA-seq data

#' ## exclude Ambigious cells
#' sce <- sce[, sce$cellType_hc != "Ambiguous"]
#' 
#' ## cellType_layer contains the final layer cell type annotations
#' table(sce$cellType_layer)
#' # Astro    EndoMural        Micro        Oligo          OPC   Excit_L2/3 
#' #  3979         2157         1601        10894         1940           82 
#' # Excit_L3 Excit_L3/4/5     Excit_L4     Excit_L5   Excit_L5/6     Excit_L6 
#' #    10459         3043         2388         2505         2487         1792 
#' # Excit_ambig        Inhib 
#' #        2053        11067 

[lahuuki]