docs: add a migration guide for v3

[baywet]

Modeled after this migration guide docs\upgrade-guide-2.md

Should indicate as a warning that Aspnet.net <10 is version 1.6, == 10 is version 2 and people using aspnet integration should ensure versions are aligned to avoid issues.

Can be based off the public API export and the OpenAPI changelog to explain all the new things and breaking changes!