[docs] Migration guide for Figma Kit

[joserodolfofreitas]

Summary

We must document how users migrate from one version to the next. Do we already have such documentation somewhere?

We god a feedback from a user regarding the update of the last figma file (v7.2.0):

"I'm looking for detailed info on how to import existing color themes into the updated file. Everything related to setting the colors appears to be different from 5.6 but I can't find any documentation."

Search keywords: docs migration

[oliviertassinari]

This is a recurring question, first saw it around 2022. The solution was to create docs for it. First in Notion, then letter moved here https://mui.com/material-ui/design-resources/material-ui-for-figma/#using-new-design-kit-versions.