Feature: Allow Sidebar tab prop to accept array

[greghesp]

https://use.docs.page/configuration#sidebar

By allowing an array over a string, it makes it possible to include or exclude the sidebar item on multiple tabs.

For instance, if I have 3 tabs "Backend, Frontend, Debugging"

I may want a Contribution group to appear on Backend and Frontend, but not Debugging. As it is only a string that allows inclusion, i must duplicate the group object and add a tab of backend and frontend, or I must accept the group shows on all 3 tabs