docs(readme.md): updated 404s in readme links (#6480)
docs(readme.md): updated 404s in readme links (#6480)

* docs(readme.md): updated 404s in readme links

* updated "Kubernetes Operator" link

updating per discussion in https://github.com/spinnaker/spinnaker/pull/6480/
spinnaker/spinnaker,spinnaker/spinnaker,spinnaker/spinnaker,spinnaker/spinnaker