Add FreeBSD entry in src/doc/rustc/src/SUMMARY.md

Author: asomers

src/doc/rustc/src/SUMMARY.md

@@ -79,6 +79,7 @@
     - [\*-nto-qnx-\*](platform-support/nto-qnx.md)
     - [*-unikraft-linux-musl](platform-support/unikraft-linux-musl.md)
     - [*-unknown-hermit](platform-support/hermit.md)
+    - [*-unknown-freebsd](platform-support/freebsd.md)
     - [\*-unknown-netbsd\*](platform-support/netbsd.md)
     - [*-unknown-openbsd](platform-support/openbsd.md)
     - [*-unknown-redox](platform-support/redox.md)