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/plugin/cable.xml

@@ -27,7 +27,7 @@
 
   <compiler autolimits="true"/>
 
-  <size nconmax="100" njmax="300" nstack="500000"/>
+  <size memory="2M"/>
 
   <worldbody>
     <composite type="cable" curve="s" count="41 1 1" size="1" offset="-.3 0 .6" initial="none">
@@ -53,4 +53,4 @@
   <actuator>
     <motor site="S_last" gear="0 0 0 1 0 0" ctrlrange="-.03 .03"/>
   </actuator>
-</mujoco>
+</mujoco>