Document how to add a custom TypeConverter for AxHost.State serialization

In order to avoid `BinaryFormatter` you need a custom `TypeConverter` for `AxHost.State` to serialize without it. We have an existing converter `AxHost.StateConverter` that does this, but it isn't actually registered for `AxHost.State`. We may decide to register it eventually, but even if we do, to support cross compilation with .NET Framework it's necessary to know how to inject a `TypeConverter` at runtime versus the normal way of applying an attribute.

`TypeConverter` docs should be updated with this how-to that we then refer to in our docs. Afaik it isn't described anywhere.