Rework doc generation to simplify and centralize

Just do all doc generation in the hack::util::gen-docs instead of spread
around. We also only track the generated docs in a single file for the
whole tree.

docs/user-guide/kubectl/.files_generated

@@ -1,31 +0,0 @@
-kubectl.md
-kubectl_annotate.md
-kubectl_api-versions.md
-kubectl_attach.md
-kubectl_cluster-info.md
-kubectl_config.md
-kubectl_config_set-cluster.md
-kubectl_config_set-context.md
-kubectl_config_set-credentials.md
-kubectl_config_set.md
-kubectl_config_unset.md
-kubectl_config_use-context.md
-kubectl_config_view.md
-kubectl_create.md
-kubectl_delete.md
-kubectl_describe.md
-kubectl_exec.md
-kubectl_expose.md
-kubectl_get.md
-kubectl_label.md
-kubectl_logs.md
-kubectl_namespace.md
-kubectl_patch.md
-kubectl_port-forward.md
-kubectl_proxy.md
-kubectl_replace.md
-kubectl_rolling-update.md
-kubectl_run.md
-kubectl_scale.md
-kubectl_stop.md
-kubectl_version.md