Update links to the Katacoda tutorials to point to the official versions
at https://katacoda.com/tektoncd and update the dashboard scenario naming
to be consistent with other (i.e. drop the redundant `tekton-` from the
name + URL).

Update the Dashboard katacoda tutorial to use the latest releases:
- Dashboard 0.13.0
- Pipelines 0.20.0

Update screenshots to reflect the new design in recent dashboard versions.
While doing this I converted the images to JPEG for significant filesize
savings over the existing PNG images (>50% on average)