Add mujoco/experimental and move USD work there.

PiperOrigin-RevId: 740730698
Change-Id: If9c68798cda502d6d7632906a034437e2557802e

src/experimental/usd/plugins/mjcf/mujoco_to_usd.h

@@ -0,0 +1,32 @@
+// Copyright 2025 DeepMind Technologies Limited
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef MUJOCO_SRC_EXPERIMENTAL_USD_PLUGINS_MJCF_MUJOCO_TO_USD_H_
+#define MUJOCO_SRC_EXPERIMENTAL_USD_PLUGINS_MJCF_MUJOCO_TO_USD_H_
+
+#include <mujoco/mujoco.h>
+#include <pxr/usd/sdf/abstractData.h>
+
+namespace mujoco {
+namespace usd {
+// Given an mjSpec, write it to a SdfAbstractData.
+//
+// Args:
+//   spec: mjSpec built programmatically or via parsed XML.
+//   data: SdfAbstractDataRefPtr that will be written to.
+bool WriteSpecToData(mjSpec* spec, pxr::SdfAbstractDataRefPtr& data);
+}  // namespace usd
+}  // namespace mujoco
+
+#endif  // MUJOCO_SRC_EXPERIMENTAL_USD_PLUGINS_MJCF_MUJOCO_TO_USD_H_