Improve Rust documentation

[felixhekhorn]

more serious follow-up from #377

When convincing people to adopt PineAPPL (such as @samiyrj for https://arxiv.org/abs/2603.09646) the number of TODO is a bit embarrassing 🙈 We were trying to copy yadism, but of course that only works up to a point ...

(there and everywhere inside where it already says TODO); same holds for e.g. ConvType

However, unfortunately these are mostly new things in PineAPPL v1 so I'm not 100% firm myself on what they exactly mean ... e.g. what does the argument of Kinematics::Scale refer to? or ScaleFuncForm::Scale?

[Radonirinaunimi]

That's a really good point and we should really put some efforts in addressing this. I will try to improve this in the next days.

[cschwan]

This was postponed as the implementation of version-1.0 functionality was prioritized; that being done we should really fill in the blanks, @Radonirinaunimi we should start with the types that are needed for Grid::new.

@felixhekhorn: I added some preliminary documentation in commit 7c03a55.