Remove unsupported Metadata component from the docs

[shannonbradshaw]

kubeflow/metadata appears to be dropped from KF:

• UI: feat: remove outdated metadata UI manifests community-distribution#1619
• Backend: feat: remove kf metadata server community-distribution#1638
• Centraldashboard entries: Remove metadata link from centraldashboard kubeflow#5404
  Do we agree that it should be dropped from the docs as well?

[shannonbradshaw]

cc: @RFMVasconcelos

[Bobgy]

+1, we should remove it.

Pipelines WG will start adding new metadata doc as part of Pipelines.

/Cc @kubeflow/wg-pipeline-leads

[thesuperzapper]

@Bobgy did you make a tracking issue to add metadata docs under the pipelines section?

I am a bit worried that users will never see it, given how many docs are in the pipelines section.