Nitpick: doc for generated vec-like methods have extra spaces

[Artoria2e5]

The current doc-generation code looks like:
https://github.com/lumol-org/soa-derive/blob/d16585c/soa-derive-internal/src/vec.rs#L54-L56

As a result, it introduces extra spaces due to markdown translating newlines to spaces:

http://lumol.org/soa-derive/soa_derive_example/struct.ParticleVec.html#impl-2

With rust-lang/rust#78837 I think a bit of extra magic can be used here to generate the entire part inside of the square brackets.

[Luthaf]

I dit not know about this feature, it seems nice! I'm not sure if changing the doc to this new use of attributes can be done without bumping MSRV though.