Update PBMC case studies documentation

[danielsarj]

Summary

This PR updates documentation across all three PBMC case studies.

Changes

All case studies

• Converted #rule-descriptions and #examine-the-output-directory-structure sections to bullet points for improved readability
• Added missing anndata_threads and anndata_mem_gb arguments to the example config files

PBMC_GEX.md

• Added a new section at the bottom with instructions for loading the final AnnData object into R for analysis with Seurat

PBMC_3k_multiome.md

• Fixed a bug in step 2 of #set-up-the-input-files where the placeholder path /path/to/fastqs/ was not removed from pbmc_unsorted_3k_library.csv