Documentation links are outdated

Documentation links on crates.io and docs.rs are outdated:
- https://crates.io/crates/ncollide2d => `Documentation` button
- https://crates.io/crates/ncollide3d => `Documentation` button
- https://www.ncollide.org/rustdoc/ncollide2d => "The rustdoc documentation is available **here**"

## Solutions

Either:
- make https://www.ncollide.org/rustdoc/ncollide available again
- or update links pointing to the documentation:
- [`Cargo.toml` for `ncollide2d` here](https://github.com/rustsim/ncollide/blob/master/build/ncollide2d/Cargo.toml#L7)
- [`Cargo.toml` for `ncollide3d` here](https://github.com/rustsim/ncollide/blob/master/build/ncollide3d/Cargo.toml#L7)
- [`src/lib.rs` here](https://github.com/rustsim/ncollide/blob/master/src/lib.rs#L12)