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/Cargo.toml

@@ -18,6 +18,7 @@ palette = { version = "0.7.3", default-features = false, features = ["std"] }
 pathfinder_color = "0.5"
 rust-embed.workspace = true
 serde.workspace = true
+serde_json.workspace = true
 simplelog = "0.9"
 strum = { version = "0.25.0", features = ["derive"] }
 theme = { path = "../theme", features = ["importing-themes"] }