docs: usage: add benchmark page

Previously, benchmark crud was not contained in the usage pages
at all.

docs/usage.rst

@@ -52,6 +52,7 @@ Usage
    usage/tar
    usage/serve
    usage/lock
+   usage/benchmark
 
    usage/help
    usage/debug

docs/usage/benchmark.rst

@@ -0,0 +1 @@
+.. include:: benchmark_crud.rst.inc