Device Path Construction: Further improve tooling and guidance

[phip1611]

#1799 was already a step into the right direction. But as we have

• DevicePathBuilder using an external buffer
• DevicePathBuilder::push only accepting single build nodes
• DevicePath::append_node() suddenly returning an allocated device path
• (soon) DevicePathBuilderOwner (xxx add OwnedDevicePathBuilder #1806)

we should streamline the interop, the documentation and also figure out what the recommended way is. For example:

• DevicePathBuilder should probably also have some kind of ::push_nodes(dv: &DevicePath)
• We should clarify when we use Box<DevicePath> and when PoolDevicePath