SUMMARY: provide links to mainline and next for Rust code docs

This mimics what we do for the normal docs in the two links above, and
while we also provide the documentation for each main release, the links
to mainline and `next` are the most useful ones, especially nowadays,
for most developers.

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

Author: ojeda

SUMMARY.md

@@ -59,7 +59,8 @@
 
   - [Kernel documentation (mainline)](https://docs.kernel.org/rust/)
   - [Kernel documentation (next)](https://docs.kernel.org/next/rust/)
-  - [Rust code documentation (v6.8)](https://rust.docs.kernel.org/6.8/)
+  - [Rust code documentation (mainline)](https://rust.docs.kernel.org)
+  - [Rust code documentation (next)](https://rust.docs.kernel.org/next/)
   - [Out-of-tree module template](https://github.com/Rust-for-Linux/rust-out-of-tree-module)
 
 ## Main branches