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/feedback/src/feedback_modal.rs

@@ -7,8 +7,8 @@ use db::kvp::KEY_VALUE_STORE;
 use editor::{Editor, EditorEvent};
 use futures::AsyncReadExt;
 use gpui::{
-    div, red, rems, serde_json, AppContext, DismissEvent, EventEmitter, FocusHandle, FocusableView,
-    Model, PromptLevel, Render, Task, View, ViewContext,
+    div, red, rems, AppContext, DismissEvent, EventEmitter, FocusHandle, FocusableView, Model,
+    PromptLevel, Render, Task, View, ViewContext,
 };
 use isahc::Request;
 use language::Buffer;