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