Improve KFP backend and installation instructions

[StefanoFioravanzo]

Since now we have Argo KFP and Tekton KFP both living in the same repo:

• https://github.com/kubeflow/pipelines/issues/10488

We need to improve the overview part, the installation flow and the architecture behind it.

cc @rimolive @Tomcli @chensun @hbelmiro @diegolovison

[hbelmiro]

This should be done after https://github.com/kubeflow/website/issues/3714.

[rimolive]

Also related. https://github.com/kubeflow/pipelines/issues/10980