Write migration instructions for v0.30

[madsmtm]

Fixes #3477.

I've not yet finished the issue description for the feedback issue, my WIP on that is below, will update this description later.

# Feedback on migration to trait-based design

We're interested in [...]

See https://github.com/rust-windowing/winit/issues/3432 for more details.

[kchibisov]

I was actually doing the same based on my PR which made the sections, you can look into how it was done

#3614

[daxpedda]

I think I would prefer more sub-headers and separating the migration from the regular changelog entries but still keep it in the same file.

[kchibisov]

ping @madsmtm @daxpedda with the context of #3614

I can finish myself, since anything that is not overly verbose/informal is fine.

[madsmtm]

I've merged this into #3614.