Add features to docs.rs config to generate proper documentation

[Sitin]

Right now certain parts of documentation for optional features are not generated. For, example, the integration is missing.

We can update build/salva3d/Cargo.toml and build/salva2d/Cargo.toml:

[package.metadata.docs.rs]
# Features to include into `docs.rs` documentation
features = [
    "rapier",
    # ...
]