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

test/engine/testdata/collision_convex/stacked_boxes.xml

@@ -9,7 +9,7 @@
     <map force="0.1"/>
   </visual>
 
-  <size njmax="3000" nconmax="1000"/>
+  <size memory="100M"/>
 
   <asset>
     <mesh name="box" vertex="-1 -1 -1 1 -1 -1 1 1 -1 1 1 1 1 -1 1 -1 1 -1 -1 1 1 -1 -1 1"