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/network/mod.rs

@@ -1 +1,3 @@
+//! Networking utilities for streaming HTTP responses.
+
 pub mod event_source;