Add note about Next.js in custom feedback UI

[hbmartin]

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Adds documentation for Next.js to avoid unhelpful webpack error when implementing custom feedback UI.
getsentry/sentry-javascript#15977

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[Lms24]

I'll leave the call up to the replay/feedback team but IMHO this is not nextJS specific but most snippets need to contextualize better where feedbackIntegration gets called and where getFeedback should be used. In general, this shouldn't differ for NextJS vs other frameworks.

[ryan953]

I just commented in getsentry/sentry-javascript#15977 (comment) but I'll add some notes here too.

As @Lms24 said, we have a NextJS specific example file here where framework specific messages and examples should live: https://github.com/getsentry/sentry-docs/blob/master/platform-includes/user-feedback/manual-injection/javascript.nextjs.mdx#L35.
In https://github.com/getsentry/sentry-docs/pull/13247/files I'm trying to make that example clearer so if someone is looking at NextJS specific instructions they'll see the right stuff, including notes about SSR and all that fun stuff.

The important thing is that users need to pick the right framework in the left side, but it scrolls away so quickly it's easy to be looking at something that's not as specific as it should be. I think we can do something about this and i'll bring it back to the team.

So for now, I think we should go with better code examples for Browser JS and NextJS, and not merge this into the general 'javascript' file.