Add react-router's NavLink ref issue to MUI "Caveat with refs" Guide

[robertvasile]

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Summary 💡

Quoting @oliviertassinari in #15903,

[There] is an issue with the NavLink component. This react-router component should forward its ref, it doesn't yet: remix-run/react-router#6056 (comment). I hope https://material-ui.com/guides/composition/#react-router-demo helps to explain why in more details.

Examples 🌈

const ForwardNavLink = React.forwardRef((props, ref) => ( <NavLink {...props} innerRef={ref} /> ));

Motivation 🔦

this should be added to https://material-ui.com/guides/composition/#caveat-with-refs

[oliviertassinari]

We already document it: https://material-ui.com/getting-started/faq/#how-do-i-use-react-router.