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

diff --cc src/liballoc/lib.rs
Simple merge

diff --cc src/libcore/lib.rs
Simple merge

diff --cc src/libcore/mem.rs
Simple merge

diff --cc src/libstd/lib.rs
Simple merge