docs: move doc generation scripts to subdir

Signed-off-by: David Karlsson <david.karlsson@docker.com>

docs/generate/go.mod

@@ -1,4 +1,4 @@
-module github.com/docker/cli/docs
+module github.com/docker/cli/docs/generate
 
 // dummy go.mod to avoid dealing with dependencies specific
 // to docs generation and not really part of the project.
@@ -10,4 +10,4 @@ go 1.16
 //	github.com/docker/cli-docs-tool v0.5.0
 //)
 //
-//replace github.com/docker/cli v0.0.0+incompatible => ../
+//replace github.com/docker/cli v0.0.0+incompatible => ../../