Better memory setting and reporting, updated example models.

- Memory heuristic rounds up to the nearest MB.
- `simulate` reports total memory currently allocated in the info box.
- All example models use `<size memory/>` rather than legacy attributes.
- Regenerated `example_saved.xml` and reformatted original model in [overview chapter](https://mujoco.readthedocs.io/en/latest/overview.html?#examples).
- `<size>` top-level section does not get written if empty.

PiperOrigin-RevId: 486702670
Change-Id: I0306e335f32344c59562ba606664ec9d2101f8a8

src/xml/xml_native_writer.cc

@@ -847,6 +847,9 @@ void mjXWriter::Size(XMLElement* root) {
   WriteAttrInt(section, "nuser_tendon", model->nuser_tendon, 0);
   WriteAttrInt(section, "nuser_actuator", model->nuser_actuator, 0);
   WriteAttrInt(section, "nuser_sensor", model->nuser_sensor, 0);
+
+  // remove entire section if no attributes
+  if (!section->FirstAttribute()) root->DeleteChild(section);
 }