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/client/src/telemetry.rs

@@ -1,10 +1,11 @@
 use crate::{TelemetrySettings, ZED_SECRET_CLIENT_TOKEN, ZED_SERVER_URL};
 use chrono::{DateTime, Utc};
 use futures::Future;
-use gpui::{serde_json, AppContext, AppMetadata, BackgroundExecutor, Task};
+use gpui::{AppContext, AppMetadata, BackgroundExecutor, Task};
 use lazy_static::lazy_static;
 use parking_lot::Mutex;
 use serde::Serialize;
+use serde_json;
 use settings::{Settings, SettingsStore};
 use std::{env, io::Write, mem, path::PathBuf, sync::Arc, time::Duration};
 use sysinfo::{