Create guides for GTEx and TCGA data

[lcolladotor]

The GTEx and TCGA datasets are some of the most used datasets. With the potential reduced memory usage via #11, it might become feasible to have user guides (vignettes) that use a subset of the GTEx or TCGA datasets. This would be helpful as those two data_sources provide different metadata than what SRA provides. In particular, there have been multiple questions from users on how to integrate the recount3 data with the TCGA metadata as TCGA itself changed sample IDs at some point. Documenting this process will be useful for users interested in these datasets.