[DashboardLayout] Provide slots for full customization of layout header and sidebar

[apedroferreira]

Summary

As briefly mentioned in the discussions in #4229 and #3750, providing slots for overriding the layout header and sidebar would be useful.

Motivation

Allowing full customization/overriding of the layout header and sidebar would give users more flexibility and provide more solutions to different problems creating custom layouts.

Search keywords: dashboard layout slots header sidebar navigation

[apedroferreira]

Possible benchmark to improve customizability in sidebar and possible new features:
https://ui.shadcn.com/docs/components/sidebar
https://x.com/shadcn/status/1847359896557408461
https://www.reddit.com/r/nextjs/comments/1gcdzrt/the_new_shadcn_sidebars_are_so_fun_to_work_with/

[apedroferreira]

Closed in favor of #4071 (duplicate but more complete spec)