Update installation/secrets to clarify registry authentication certs and cover shared-secrets

Prevent confusion of the Registry's authentication certicates with SSL certificates, by placing their generation in a separate section.

Closes #257 (closed)

Update doc/installation/secrets.md with information regarding the automatic behaviors of the shared-secrets job. Let the user know that they don't have to manually complete these tasks, unless they choose to provide custom values.

Closes #262 (closed)