SUMMARY: add "Tools" category

This new category is intended for tools and toolchains (such as Coccinelle
for Rust and `rustc_codegen_gcc`) that will have their own subpages.

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

SUMMARY.md

@@ -20,6 +20,8 @@
   - Hidden [The Safe Pinned Initialization Problem](The-Safe-Pinned-Initialization-Problem.md)
   - Hidden [Ksquirrel](Ksquirrel.md)
 
+## Tools
+
 ## Users
 
   - [NVMe Driver](NVMe-driver.md)