Improve `package.metadata.docs.rs.rustdoc-args`

Set `package.metadata.docs.rs.rustdoc-args` like following:

```toml
rustdoc-args = [
    "--generate-link-to-definition",
    "--generate-macro-expansion",
    # std-deps:
    "--extern-html-root-url=core=https://doc.rust-lang.org",
    "--extern-html-root-url=alloc=https://doc.rust-lang.org",
    "--extern-html-root-url=std=https://doc.rust-lang.org",
]
```