Next iteration of kubernetes-reference-RI docs

[bwgartner]

Updates/renaming of image files

Cleanup of unused files

Referencing newer product versions of

• Rancher/K3s/RKE1/RKE2/K8s with updates for SLE 15SP3 and SLE Micro 5.1

Updated Doctype RI

• for four existing ones and new RKE1/RKE2 variants can be published

After a review, feel free to merge this and publish all 6 DC files (4 replacements + new RKE / RKE2 ones)