docs(lib): extract feature documentation from Cargo.toml

[orhun]

Utilize document-features crate to extract the feature documentation from Cargo.toml

before / after:

One thing to note, we can possibly make it an optional dependency: slint-ui/document-features#8

Let me know what you think 🐻

[codecov]

Codecov Report

Merging #438 (8bc8d5c) into main (b35f19e) will increase coverage by 0.39%.
Report is 2 commits behind head on main.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #438      +/-   ##
==========================================
+ Coverage   89.95%   90.34%   +0.39%     
==========================================
  Files          40       40              
  Lines       10878    10977      +99     
==========================================
+ Hits         9785     9917     +132     
+ Misses       1093     1060      -33     

see 2 files with indirect coverage changes

[Valentin271]

Saw this crate recently too. I think its a great idea, its easier to maintain like so.

As for making document-feature an optional dependency, I think it's better because users don't need to build it every time.
That would lead to a feature named document-feature which makes sense to me.
Other crates are using this pattern too like egui.

As the number of crates using document-feature grows (only 138 right now), I think people will get used to having this feature and will instinctly know what it is, like serde.

[orhun]

Made the dependency optional in 78363c5

[joshka]

The https://lib.rs changes for showing features is neat too:
https://users.rust-lang.org/t/new-features-on-lib-rs-website/98560