Migration Guide V10 -> V11

[fhammerschmidt]

Especially considering uncurried mode and RescriptCore and whatever came up in discussions: https://forum.rescript-lang.org/t/the-big-migration-thread-for-rescript-v11-and-uncurried-mode

Possible improvements:

• Changes for genType users
• React/JSX4-specific changes
• Complete changelog with breaking changes, but not split up into alpha, beta, rc versions (done in Some improvements to the v11 migration guide #762)

[fhammerschmidt]

It's partially done at https://rescript-lang.org/docs/manual/latest/migrate-to-v11-and-uncurried-mode