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

model/hammock/hammock.xml

@@ -24,7 +24,7 @@
 
   <option timestep="0.005" solver="CG" iterations="30" tolerance="1e-6"/>
 
-  <size njmax="1000" nconmax="100" nstack="1000000"/>
+  <size memory="20M"/>
 
   <visual>
     <map force="0.1" zfar="30"/>