postprocessing/upgrade_comparison README improvements

[lixiliu]

• Minor description cleanup, like "upgrade 0 is the baseline" should be for upgrades instead of s3_results_dir
• Remove resstock/postprocessing> from example CLI cmd
• Add link to SSO instruction
• Add link to shared setup for postprocessing
• Clarity of options when --help is called:
  • Denotes the default in the description for all optional flags
  • Show example of the QUATITY_GROUP
• I am not sure what upgrade 1 is in the CI workflow yaml but if it is not already an electrification upgrade, can we pick an upgrade that is?
  cc: @rajeee