update v2 migration docs about babel configuration

[thescientist13]

Summary

Updates v2 migration guide with a section about potential conflicts with root level .babelrc configuration files

• discussed here here
• advised on the approach here

[m-allanson]

Nice, thank you 👍

[nihgwu]

@m-allanson I think we should avoid using temporary link https://v2--gatsbyjs.netlify.com/ in any place, I just closed a similar PR #5870

[jlengstorf]

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