Update documentation about deprecating images with Tornjak and SPIRE packaged together.

[mrsabath]

Since the PR #97 Tornjak can no longer be packed along the SPIRE container in a single image. SPIRE is using scratch and Tornjak requires alpine (to use shell). We separated the Tornjak code into a separate container that might be co-running with SPIRE in a single pod. All the documentation is still referencing the old image with SPIRE and Tornjak together. Update the documentation with references to the new deployment model