Docs: Update developing zed docs to match (#21379)

Some changes just so the build docs for the different os matches each
other :)

macos:
- moved `rust wasm toolchain install` up under `rust install` (match
windows docs)
- add instructions to update rust if already installed (match windows
and linux docs)

windows:
- add `(required by a dependency)` to cmake install (match macos docs)

Release Notes:

- N/A

docs/src/development/linux.md

@@ -6,11 +6,7 @@ Clone down the [Zed repository](https://github.com/zed-industries/zed).
 
 ## Dependencies
 
-- Install [Rust](https://www.rust-lang.org/tools/install). If it's already installed, make sure it's up-to-date:
-
-  ```sh
-  rustup update
-  ```
+- Install [rustup](https://www.rust-lang.org/tools/install)
 
 - Install the necessary system libraries: