docs: rust: Move testing to a separate page

To be able to add more testing documentation move the testing
section to it's own page.

No change on the documentation itself.

Suggested-by: Trevor Gross <tmgross@umich.edu>
Suggested-by: Miguel Ojeda <ojeda@kernel.org>
Reviewed-by: Trevor Gross <tmgross@umich.edu>
Reviewed-by: David Gow <davidgow@google.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Dirk Behme <dirk.behme@de.bosch.com>
Link: https://lore.kernel.org/r/20240130075117.4137360-1-dirk.behme@de.bosch.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>