fix: initialize git submodules in docs workflow

The tree-sitter-gram submodule was not being fetched, causing
cargo metadata to fail with a missing Cargo.toml error.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: akollegger

.github/workflows/docs.yml

@@ -17,7 +17,9 @@ jobs:
   build:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
+        with:
+          submodules: true
 
       - name: Setup Rust (stable)
         uses: dtolnay/rust-toolchain@stable