On some pages we have a splitted layout for which we use `Primer::Alpha::Layout`, e.g.

* Meetings

* Storages


For the right side pane (called `Sidepanel`), I want a fixed component which can be re-used so that all right panes look the same

### Acceptance criteria

* [x] Create a Sidebar component
* [x] Provide slots for individual secions with
* [x] a header
* [x] a counter
* [x] an action button
* [x] some content
* [x] an optional footer
* [x] Use the `Primer::OpenProject::BorderGrid` internally

#### Hint for QA

* This will be implemented completely inside the primer\_view\_components Begin of the insertionrepo. It canEnd of the insertion Begin of the deletion repo, and this needs toEnd of the deletion be tested Begin of the insertiononceEnd of the insertion Begin of the deletion insideEnd of the deletion the Begin of the insertionchild implementation tickets are merged by havingEnd of the insertion Begin of the deletion lookbook (or in core after it was updated there, which will be probably beEnd of the deletion a Begin of the insertionlook at those places.End of the insertion Begin of the deletion separate ticket).End of the deletion