[rustdoc] Box ItemKind to reduce the size of Item

[jyn514]

This brings the size of Item from

[src/librustdoc/lib.rs:103] std::mem::size_of::<Item>() = 536

to

[src/librustdoc/lib.rs:103] std::mem::size_of::<Item>() = 136

This is an alternative to #79967; I don't think it makes sense to make both changes.

Helps with #79103.