Add in-code docs for Rust

[xushiyan]

No description provided.

[KnightChess]

does it meat add doc for every Moudles, Sturct, Method in code,
like this?

/// Table interface for engine integrate
#[derive(Clone, Debug)]
pub struct Table {
    pub base_url: Arc<Url>,
    pub configs: Arc<HudiConfigs>,
    pub extra_options: Arc<HashMap<String, String>>,
    pub timeline: Timeline,
    pub file_system_view: FileSystemView,
}

[xushiyan]

@KnightChess that's correct. the goal is to make https://docs.rs/hudi/latest/hudi/ have docs

[KnightChess]

@xushiyan got it. I've been familiarizing myself with this project recently, and I can help contribute to this document.