Improve docs on migration script

[aalves08]

Spin-off of #12656, just do tackle the documentation part:

https://extensions.rancher.io/extensions/next/rancher-2.10-support#how-to-proceed-with-your-extension-update

Docs

• After migration tool
  • Missing step to update local node version to 20 <= local node <= 22
  • Missing step to do a yarn install
  • Add info on changing TS components that import JS components typing
    • for example component: { Loading } needs to be component: { Loading } --> as any