doc: man-page for if

Provide a man-page for the if command.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
diff --git a/doc/usage/index.rst b/doc/usage/index.rst
index 0d174ee..66d73e7 100644
--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -71,6 +71,7 @@
    cmd/gpt
    cmd/history
    cmd/host
+   cmd/if
    cmd/imxtract
    cmd/load
    cmd/loadb