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

[phip1611]

btw, not directly related but I don't want to forget it:

diff --git a/uefi/src/boot.rs b/uefi/src/boot.rs
index c6449a42..683fe51c 100644
--- a/uefi/src/boot.rs
+++ b/uefi/src/boot.rs
@@ -48,6 +48,7 @@ use core::{mem, slice};
 use uefi_raw::table::boot::{AllocateType as RawAllocateType, InterfaceType, TimerDelay};
 #[cfg(feature = "alloc")]
 use {alloc::vec::Vec, uefi::ResultExt};
+use crate::fs::FileSystem;
 
 /// Global image handle. This is only set by [`set_image_handle`], and it is
 /// only read by [`image_handle`].
@@ -1477,8 +1478,11 @@ pub fn stall(duration: Duration) {
     }
 }
 
-/// Retrieves a [`SimpleFileSystem`] protocol associated with the device the given
-/// image was loaded from.
+/// Retrieves a [`SimpleFileSystem`] [`Protocol`] associated with the device the
+/// given image was loaded from.
+///
+/// You may want to wrap the returned value in [`FileSystem`] for a more
+/// convenient experience.
 ///
 /// # Errors
 ///