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/tendon_arm/arm26.xml

@@ -16,8 +16,6 @@
 <mujoco model="2-link 6-muscle arm">
   <option timestep="0.005" iterations="50" solver="Newton" tolerance="1e-10"/>
 
-  <size njmax="50" nconmax="10" nstack="200"/>
-
   <visual>
     <rgba haze=".3 .3 .3 1"/>
   </visual>