docs: rust: add "The Rust experiment" section

Clarify that the Rust experiment is still going on to avoid
confusion for both kernel maintainers and end users.

Reviewed-by: Carlos Bilbao <carlos.bilbao@amd.com>
Link: https://lore.kernel.org/r/20231018160922.1018962-1-ojeda@kernel.org


[ Changed last paragraph as discussed in the mailing list. ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>