Combine "SITE" and "CRED" key discussions. Add "SIGN" key
There are several similarities in how you manage CIVICRM_SITE_KEY and CIVICRM_CRED_KEYS (e.g. find civicrm.settings.php
; use a script to generate random value; make sure it's long enough). The CIVICRM_SIGN_KEYS in 5.36 will also be similar.
This consolidates the documentation for all three into the same page.