Docs: Allow https://nextjs.org/ to work when PerformanceObserver is undefined

[danielhjacobs]

What is the improvement or update you wish to see?

Don't crash when PerformanceObserver is undefined.

Is there any context that might help us understand?

If you go into about:config on the latest version of Firefox and set dom.enable_performance_observer to false, https://nextjs.org/ crashes with the error message Application error: a client-side exception has occurred (see the browser console for more information). and the console log PerformanceObserver is not defined. This is #64211 recreated in a modern "evergreen" browser.

Does the docs page already exist? Please link to it.

https://nextjs.org/