chore(rust): add description for rolldown_common (#702)

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### Description

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### Test Plan

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---

crates/rolldown_common/Cargo.toml

@@ -1,6 +1,7 @@
 [package]
-name    = "rolldown_common"
-version = "0.0.1"
+description = "This crate is mostly for sharing code between `rolldown` and `rolldown_plugin`"
+name        = "rolldown_common"
+version     = "0.0.1"
 
 edition.workspace    = true
 homepage.workspace   = true

crates/rolldown_common/src/lib.rs

@@ -1,3 +1,5 @@
+//! `rolldown_common` is mostly for sharing code between `rolldown` and `rolldown_plugin`. This means only `rolldown` and `rolldown_plugin`
+//! could rely on this crate. Other `rolldown_xxx` crates should not rely on this crate but other way around.
 mod module;
 mod types;