Move rust code documentation link from SUMMARY to Branches

It has been more than a year since the `rust` branch was archived,
thus move the documentation link.

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

Author: ojeda

SUMMARY.md

@@ -60,7 +60,6 @@
   - [Kernel documentation (mainline)](https://docs.kernel.org/rust/)
   - [Kernel documentation (next)](https://docs.kernel.org/next/rust/)
   - [Rust code documentation (v6.8)](https://rust-for-linux.github.io/docs/v6.8/kernel/)
-  - [Rust code documentation (`rust` 2023-03-13)](https://rust-for-linux.github.io/docs/rust/kernel/)
   - [Out-of-tree module template](https://github.com/Rust-for-Linux/rust-out-of-tree-module)
 
 ## Main branches

src/Branches.md

@@ -84,4 +84,6 @@ It contains most of the abstractions that the project worked on as a prototype/s
 
 The branch is now archived, thus no new changes are merged into it. While it may be deleted eventually, for the moment it is kept around since some of the code did not make it upstream and may be useful for others.
 
+Similarly, its [Rust code documentation (2023-03-13)](https://rust-for-linux.github.io/docs/rust/kernel/) is archived as well.
+
 Changes to this branch landed via [GitHub PRs](https://github.com/Rust-for-Linux/linux/pulls). GitHub Actions was used as a pre-merge CI, compiling the kernel and booting it under QEMU for different toolchains, architectures and configurations. It also checked that some tests passed (e.g. loading sample modules, KUnit tests...) as well as building the PR under Clippy, building the docs, checking `rustfmt`, etc. [KernelCI](https://linux.kernelci.org/job/rust-for-linux/branch/rust/) tests it. Finally, in the past, the [Ksquirrel](Ksquirrel.md) bot checked the PRs sent to it.