Write upgrade guide for previous iterations to Stage 3

[pzuraq]

Decorators have mostly stabilized, and the end result is different than previous iterations. We should add an upgrade guide (or link to an existing one) for users of the old legacy Babel/TypeScript decorators to the latest, to make it easier for the community to adopt it. The 2018 decorators were not widely adopted, so it doesn't makes much sense to invest time in writing a guide for those.