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/view.rs

@@ -1,5 +1,5 @@
 use crate::{
-    private::Sealed, AnyElement, AnyModel, AnyWeakModel, AppContext, AvailableSpace, BorrowWindow,
+    seal::Sealed, AnyElement, AnyModel, AnyWeakModel, AppContext, AvailableSpace, BorrowWindow,
     Bounds, Element, ElementId, Entity, EntityId, Flatten, FocusHandle, FocusableView, IntoElement,
     LayoutId, Model, Pixels, Point, Render, Size, ViewContext, VisualContext, WeakModel,
     WindowContext,