Replace dummy crate with actual snippet crates in workspace

Update workspace members to include the real Rust crates:
- parachains/customize-runtime/pallet-development/create-a-pallet
- reference/tools/subxt

Remove the placeholder dummy crate that was only added to satisfy CI.

Author: brunopgalvao

.snippets/code/Cargo.toml

@@ -10,7 +10,10 @@ repository = "https://github.com/paritytech/polkadot-sdk-parachain-template.git"
 edition = "2021"
 
 [workspace]
-members = ["dummy"]
+members = [
+    "parachains/customize-runtime/pallet-development/create-a-pallet",
+    "reference/tools/subxt",
+]
 resolver = "2"
 
 [workspace.dependencies]