docs: add library-level documentation

Add crate-level doc comment describing the purpose and available modules.

Author: drmingdrmer

src/lib.rs

@@ -1 +1,8 @@
+//! Common utilities and data structures for Databend.
+//!
+//! # Modules
+//!
+//! - [`histogram`]: A histogram with logarithmic bucketing for tracking u64 value distributions.
+//!   Provides O(1) recording and efficient percentile calculation with bounded memory (~2KB).
+
 pub mod histogram;