Sidepanel and summary items

[NotMyFault]

Describe your use-case which is not covered by existing documentation.

Picking up the idea from jenkinsci/jenkins#6918 (review), to define pointers how to use sidebar and mainpanel items, instead of duplicating one into each other.

Reference any relevant documentation, other materials or issues/pull requests that can be used for inspiration.

No response

[daniel-beck]

FTR https://gitter.im/jenkinsci/ux-sig?at=62de6a1def5ee44882eb8f3a (IMO we should look into a more substantial redesign of the main job/build pages).