docs: Added reminder for menu import to rendition menu docs

[JeroenReumkens]

I did not just lose 4 hours of my life trying to find out why the rendition menu wasn't working 🥲

To prevent someone else from making the same mistake, I added this warning. Copied it from other menu pages that already included this warning, which I only encountered after reverse engineering how the menu works myself 😅

Hope it's helpful to at least one person out there!

[vercel[bot]]

@JeroenReumkens is attempting to deploy a commit to the Mux Team on Vercel.

A member of the Team first needs to authorize it.

[bhatvikrant]

🫡