device: rust: expand documentation for Device

The documentation for the generic Device type is outdated and deserves
much more detail.

Hence, expand the documentation and cover topics such as device types,
device contexts, as well as information on how to use the generic device
infrastructure to implement bus and class specific device types.

Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20250722150110.23565-3-dakr@kernel.org


[ Add empty line after code blocks, "in" -> "within", remove unnecessary
  pin annotations in class device example. - Danilo ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>