Auto merge of #53227 - nivkner:pin_move, r=RalfJung

move the Pin API into its own module for centralized documentation

This implements the change proposed by @withoutboats in #49150, as suggested by @RalfJung in the review of #53104,
along with the documentation that was originally in it, that was deemed more appropriate in module-level documentation.

r? @RalfJung

src/liballoc/lib.rs

@@ -160,6 +160,7 @@ pub mod collections;
 pub mod sync;
 pub mod rc;
 pub mod raw_vec;
+pub mod pin;
 pub mod prelude;
 pub mod borrow;
 pub mod fmt;