docs: Document render prop

[devongovett]

Docs for the new render prop added in #9499. Also adds ref merging to mergeProps.

• New section in the Customization guide
• Removes RouterProvider from the framework setup guide
• Adds example of rendering router links to Menu, ListBox, and Tabs
• Adds warning to links section under Table, TagGroup, GridList, and Tree recommending using onAction with programmatic navigation. Eventually we may support rendering router links directly, but this breaks the HTML spec so we will need to test more. Since RouterProvider has a lot of issues with various routers, and these components don't use real <a> elements anyway, it's basically just programmatic navigation with extra steps. Removing this from the docs simplifies things for us and encourages users to use their framework's native API directly.

[rspbot]

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