Add Git submodule initialization to repository setup steps (#5818)

Was running into an issue building live_kit_server after a fresh clone
due to missing dependencies for build.rs. The use of git submodules
wasn't currently documented.

docs/src/developing_zed__building_zed.md

@@ -1,5 +1,13 @@
 # Building Zed
 
+## Repository
+
+After cloning the repository, ensure all git submodules are initialized:
+
+```shell
+git submodule update --init --recursive
+```
+
 ## Dependencies
 
 - Install [Rust](https://www.rust-lang.org/tools/install)