Release 0.5.0 - #[non_exhaustive] attribute

[FauconFan]

This follows multiple discussions: #1848 #1851 #1835.

As stated, this PR could be accepted when the release 0.5.0 will be landed. I've added the non_exhaustive attribute on the Chapter and Theme structs, since this is a breaking change anyway. I checked if adding non_exhaustive attribute on enums is a good practice, but in my research I have found that is not really very well handled by serde: Link here. I think this PR is good as it is.