Document what the --zed project style does

Summary

The CLI help lists three project options/styles:

-w, --workspace    Setup your project as a workspace
-z, --zed          Setup a zed style project

The README documents the default project layout and the workspace project layout, but it does not explain what "zed style" means or how --zed changes the generated project.

Current gap

A new user can see --zed in create-gpui-app --help, but the docs do not clarify:

• whether --zed is a separate template or a modifier
• whether it can be combined with --workspace
• what files/layout change when it is enabled
• what "zed style" means in practical terms

Observed behavior

From the generator implementation, --zed appears to rename/configure the binary entrypoint rather than selecting a completely separate template.

For a default project, instead of:

my_app/
  Cargo.toml
  src/
    main.rs

--zed generates/configures something like:

my_app/
  Cargo.toml
  src/
    my_app.rs

and adds an explicit bin target:

[[bin]]
name = "my_app"
path = "src/my_app.rs"

For a workspace project, the same style is applied inside the crate:

my_app/
  Cargo.toml
  crates/
    my_app/
      Cargo.toml
      src/
        my_app.rs

with the bin target path updated to src/my_app.rs.

Suggested docs addition

It would help to add a short section explaining that:

• --workspace controls whether the app is generated as a Cargo workspace
• --zed applies a Zed-style binary layout
• --zed can be combined with --workspace
• Zed style means using an explicit [[bin]] target and a source file named after the app, e.g. src/my_app.rs, instead of the conventional src/main.rs

This would make the CLI help and README consistent and reduce confusion for first-time GPUI users.