add docs to profile module

src/repo/profile/mod.rs

@@ -1,3 +1,16 @@
+//! Evaluate profiles:
+//! ```rust
+//! use gentoo_utils::repo::Repo;
+//!
+//! let repo = Repo::new("/var/db/repos/gentoo");
+//! let profile = repo.evaluate_profile("default/linux/23.0")
+//!     .expect("failed to evaluate profile");
+//!
+//! for (key, value) in profile.make_defaults() {
+//!     println!("{key} = {value}");
+//! }
+//! ```
+
 use std::{
     collections::HashMap,
     fs::{self, File},