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/src/platform/mac/display_linker.rs

@@ -94,7 +94,7 @@ unsafe extern "C" fn trampoline(
 
 mod sys {
     //! Derived from display-link crate under the fololwing license:
-    //! https://github.com/BrainiumLLC/display-link/blob/master/LICENSE-MIT
+    //! <https://github.com/BrainiumLLC/display-link/blob/master/LICENSE-MIT>
     //! Apple docs: [CVDisplayLink](https://developer.apple.com/documentation/corevideo/cvdisplaylinkoutputcallback?language=objc)
     #![allow(dead_code, non_upper_case_globals)]