Link to instructions for credentialing process

[bemoody]

At https://mimic.mit.edu/docs/gettingstarted/, there is a link "Instructions for the credentialing process are provided on PhysioNet". Currently this link points to https://physionet.org/settings/credentialing/.

However, that URL requires login and also doesn't really provide much explanation. Maybe it would be better for this link to point to https://physionet.org/about/citi-course/ instead?

(Michael also pointed out that the screenshots on that page may need updating.)