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/project_symbols/Cargo.toml

@@ -24,6 +24,7 @@ anyhow.workspace = true
 ordered-float.workspace = true
 postage.workspace = true
 smol.workspace = true
+serde_json.workspace = true
 
 [dev-dependencies]
 futures.workspace = true

crates/project_symbols/src/project_symbols.rs

@@ -260,9 +260,10 @@ impl PickerDelegate for ProjectSymbolsDelegate {
 mod tests {
     use super::*;
     use futures::StreamExt;
-    use gpui::{serde_json::json, TestAppContext, VisualContext};
+    use gpui::{TestAppContext, VisualContext};
     use language::{FakeLspAdapter, Language, LanguageConfig};
     use project::FakeFs;
+    use serde_json::json;
     use settings::SettingsStore;
     use std::{path::Path, sync::Arc};