add platform-support/openbsd.md in SUMMARY.md

Author: semarie

src/doc/rustc/src/SUMMARY.md

@@ -15,6 +15,7 @@
 - [Platform Support](platform-support.md)
     - [aarch64-apple-ios-sim](platform-support/aarch64-apple-ios-sim.md)
     - [\*-kmc-solid_\*](platform-support/kmc-solid.md)
+    - [*-unknown-openbsd](platform-support/openbsd.md)
     - [x86_64-unknown-none](platform-support/x86_64-unknown-none.md)
     - [wasm64-unknown-unknown](platform-support/wasm64-unknown-unknown.md)
 - [Target Tier Policy](target-tier-policy.md)