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/language_tools/Cargo.toml

@@ -24,6 +24,7 @@ futures.workspace = true
 serde.workspace = true
 anyhow.workspace = true
 tree-sitter.workspace = true
+serde_json.workspace = true
 
 [dev-dependencies]
 client = { path = "../client", features = ["test-support"] }

crates/language_tools/src/lsp_log_tests.rs

@@ -4,9 +4,10 @@ use crate::lsp_log::LogMenuItem;
 
 use super::*;
 use futures::StreamExt;
-use gpui::{serde_json::json, Context, TestAppContext, VisualTestContext};
+use gpui::{Context, TestAppContext, VisualTestContext};
 use language::{tree_sitter_rust, FakeLspAdapter, Language, LanguageConfig, LanguageServerName};
 use project::{FakeFs, Project};
+use serde_json::json;
 use settings::SettingsStore;
 
 #[gpui::test]