docs: rust: link to https://rust.docs.kernel.org

The Rust code documentation (i.e. `rustdoc`-generated docs) is now
available at:

    https://rust.docs.kernel.org



Thus document it and remove the `TODO` line.

The generation uses a particular kernel configuration, based on x86_64,
which may get tweaked over time. Older tags, and how they are generated,
may also change in the future. We may consider freezing them at some
point, but for the moment, the content should not be considered immutable.

Thanks Konstantin for the support setting it up!

Cc: Konstantin Ryabitsev <konstantin@linuxfoundation.org>
Acked-by: Konstantin Ryabitsev <konstantin@linuxfoundation.org>
Link: https://lore.kernel.org/r/20240818141200.386899-1-ojeda@kernel.org


Signed-off-by: Miguel Ojeda <ojeda@kernel.org>