Add rustdoc comments to all public items

Document all public structs, enums, traits, functions, fields, and
variants across the library. Adds crate-level documentation with a
usage example, module-level docs, and API reference links where
applicable. Also fixes four bare URL warnings in existing doc comments.

src/lib.rs

@@ -1,8 +1,37 @@
+//! Async Rust client for the Google Gemini API.
+//!
+//! This crate provides a high-level async client for interacting with Google's Gemini
+//! generative AI models. It supports content generation (including streaming via SSE),
+//! token counting, text embeddings, and image generation.
+//!
+//! # Usage
+//!
+//! ```no_run
+//! use google_genai::prelude::*;
+//!
+//! # async fn run() -> google_genai::error::Result<()> {
+//! let client = GeminiClient::new("YOUR_API_KEY".into());
+//!
+//! let request = GenerateContentRequest::builder()
+//!     .contents(vec![
+//!         Content::builder().add_text_part("Hello, Gemini!").build()
+//!     ])
+//!     .build();
+//!
+//! let response = client.generate_content(&request, "gemini-2.0-flash").await?;
+//! # Ok(())
+//! # }
+//! ```
+
 mod client;
 pub mod error;
 pub mod network;
 mod types;
 
+/// Convenience re-exports of the most commonly used types.
+///
+/// Importing `use google_genai::prelude::*` brings [`GeminiClient`](crate::prelude::GeminiClient)
+/// and all request/response types into scope.
 pub mod prelude {
     pub use crate::client::*;
     pub use crate::types::*;