Update PDC download instructions ICGC DCC docs site

[hknahal]

University of Chicago has removed data download via AWS CLI, so the current download instructions at https://docs.icgc.org/download/downloading-data/#downloading-data-from-pdc-repository will not work.
Junjun sent updated instructions for downloading data at PDC: https://extsd.oicr.on.ca/projects/ICGCSD/queues/custom/106/ICGCSD-5806 The following links will need to be updated:

https://docs.icgc.org/download/downloading-data/#downloading-data-from-pdc-repository
https://docs.icgc.org/pcawg/data/#download-from-pdc
https://docs.icgc.org/download/repositories/#convert-icgc-manifest-file-to-pdcs-gen3-manifest-file (make sure the Convert ICGC manifest file to PDC's Gen3 manifest file instructions are up to date)