gpui: Further docs refinement & moved some reexports into 'private' module (#3935)

This commit mostly fixes invalid URLs in docstrings. It also
encapsulates crates we reexport (serde stuff + linkme) into a public
module named "private" in order to reduce the API surfaced through docs.
Moreover, I fixed up a bunch of crates that were pulling serde_json in
through gpui explicitly instead of using Cargo manifest.

Release Notes:
- N/A

crates/gpui/tests/action_macros.rs

@@ -11,7 +11,7 @@ fn test_action_macros() {
 
     impl_actions!(test, [AnotherTestAction]);
 
-    #[derive(PartialEq, Clone, gpui::serde_derive::Deserialize)]
+    #[derive(PartialEq, Clone, gpui::private::serde_derive::Deserialize)]
     struct RegisterableAction {}
 
     register_action!(RegisterableAction);