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/theme_importer/src/main.rs

@@ -16,7 +16,6 @@ use any_ascii::any_ascii;
 use anyhow::{anyhow, Context, Result};
 use clap::Parser;
 use convert_case::{Case, Casing};
-use gpui::serde_json;
 use indexmap::IndexMap;
 use indoc::formatdoc;
 use json_comments::StripComments;