wiki: add nav.yaml This will be used for the new sidebars. Change-Id: I425f38af4497e6a2f7b8e3605dbbb84c08f80e4c Reviewed-on: https://go-review.googlesource.com/c/wiki/+/550955 Reviewed-by: Russ Cox <rsc@golang.org> Commit-Queue: Russ Cox <rsc@golang.org>