Dagger Go SDK 0.4: Multi-platform, improved filesystem API and more

The Dagger Go SDK 0.4.0 has been released, introducing several improvements including multi-platform pipelines, an improved filesystem API, and less boilerplate code. The new version supports pulling container images and executing commands on platforms that differ from the underlying host, allowing for compilation of binaries targeting different CPU architectures, testing, and pushing to a registry bundled as a multi-platform container image all on a single host. The updated API also simplifies interactions with the host filesystem by introducing `Host.Directory` and other improvements. Additionally, new guides have been added to the documentation to help users make the most of the Go SDK, covering topics such as working with the host filesystem, understanding multi-platform support, copying embedded directories into a container, and using Dagger with private Git repositories.