rust: types: add examples for the `Either` type

rust: types: add examples for the `Either` type

We aim to have examples in all Rust types, thus add basic ones for the
`Either` type.

Suggested-by: Miguel Ojeda <ojeda@kernel.org>
Signed-off-by: Nell Shamrell-Harrington <nells@linux.microsoft.com>
Tested-by: Dirk Behme <dirk.behme@de.bosch.com>
Reviewed-by: Trevor Gross <tmgross@umich.edu>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://rust-for-linux.zulipchat.com/#narrow/stream/291565/topic/x/near/467478085
Link: https://lore.kernel.org/r/20240918212052.8790-1-nells@linux.microsoft.com
[ Reworded slightly. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>