Document git submodule support in Deno Deploy builds

Split out from #2518.

A user deploying a repo that uses **git submodules** found that the submodule directories remain empty on the build server. They tried both the **Install Command** and **Build Command** fields to run `git submodule update --init`, without success.

The build docs don't currently cover submodules at all ("submodule" appears nowhere in `deploy/`). The [Build stages](https://docs.deno.com/deploy/reference/builds/#build-stages) section only says the "Preparing" stage "downloads the source code", without specifying whether submodules are initialized during checkout.

**This needs a platform answer before docs can be written:**

- Does the Deno Deploy build process check out git submodules (e.g. clone with `--recurse-submodules`)?
- If not, is there a supported way for users to initialize them (e.g. via the Install Command), or is it currently unsupported?

Once the supported behavior is confirmed, document it in the [builds reference](https://docs.deno.com/deploy/reference/builds/): either how submodules are handled automatically, or the recommended workaround / current limitation.

Original report: https://github.com/denoland/docs/issues/2518#issuecomment-3315053573

Start with the build documentation under deploy/ and the builds reference, especially the Build stages section. First confirm with the Deno Deploy platform whether checkout initializes git submodules and whether Install Command supports a workaround; then document the confirmed behavior or limitation in the builds reference.

Written by the indexing model from the issue text.