docs: add apptainer sandbox guide

[neubig]

Adds comprehensive documentation for using Apptainer-based sandboxed workspaces.

Related to OpenHands/software-agent-sdk#892

This guide demonstrates:

• When to use Apptainer vs Docker
• Configuration options for ApptainerWorkspace
• Example usage with pre-built images
• Building from base images
• Using existing SIF files
• Troubleshooting common issues

Apptainer is ideal for HPC and shared computing environments where Docker is not available or root access is restricted.

[neubig]

Thanks for the review @xingyaoww!

Regarding testing: The example follows the same pattern as the tested Docker workspace example. The code structure has been reviewed and:

• All CI checks pass in SDK#892 (except check-examples which needs this docs PR merged)
• The implementation follows the established RemoteWorkspace pattern
• Unit tests verify the API surface and class structure

Full integration testing with Apptainer would require Apptainer installation which isn't feasible in all CI environments (similar to how we don't run full Docker tests in CI for every PR).

I've also renamed the branch to openhands/apptainer-workspace-891 to match the SDK PR branch name, which will allow the check-examples workflow to find this documentation.

[xingyaoww]

@neubig Looks like we are actually able to setup-apptainer in CI 👀

https://github.com/marketplace/actions/setup-apptainer