chore(docs): make changelogs visible on docsrs

Split changelog file to make it more comprehensible when reading and
also make it a part of documentation so it'll be more discoverable
by the users. This change also makes it possible for rust code inside
the changelogs to be tested with `cargo`.

Co-authored-by: Kirill Chibisov <contact@kchibisov.com>

src/lib.rs

@@ -202,6 +202,8 @@ pub use rwh_06 as raw_window_handle;
 pub use dpi;
 
 pub mod application;
+#[cfg(any(doc, doctest, test))]
+pub mod changelog;
 #[macro_use]
 pub mod error;
 mod cursor;