doc: add module-level docs

fixes CI.

src/hex.rs

@@ -1,3 +1,5 @@
+//! A module for encoding and decoding hexadecimal strings.
+
 use crate::err;
 use terrors::OneOf;
 

src/macros.rs

@@ -1,3 +1,5 @@
+//! Macros for error handling.
+
 #[macro_export]
 macro_rules! err {
     ($item:expr) => {{

src/main.rs

@@ -1,3 +1,6 @@
+//! A simple faucet server that uses [`axum`] and [`bdk_wallet`]
+//! to generate and dispense bitcoin.
+
 pub mod hex;
 pub mod macros;