[Documentation] Update guides for the New Katib UI

[andreyvelich]

/kind feature

In Katib 0.12 release we will use the new Katib UI in the default manifests.
We should modify Kubeflow.org doc with the new Katib UI in:

1. Introduction to Katib: https://www.kubeflow.org/docs/components/katib/overview/.
2. Getting Started with Katib: https://www.kubeflow.org/docs/components/katib/hyperparameter/
3. Running an Experiment: https://www.kubeflow.org/docs/components/katib/experiment/.

In addition to that, we should modify the README: https://github.com/kubeflow/katib#web-ui.

/area release

/cc @kimwnasptd @gaocegege @johnugeorge

[kimwnasptd]

I can pick it up and work on it as part of the docs efforts for Kubeflow 1.4

/assign @kimwnasptd