Add flex and elasticity documentation. Fixes #873.

PiperOrigin-RevId: 574179040
Change-Id: Iaf90bbf2b2c3fe075c940c2b66b7b841f256e653

model/plugin/elasticity/mannequin.xml

@@ -38,7 +38,7 @@
     <default class="body">
 
       <!-- geoms -->
-      <geom type="capsule" condim="1" friction=".7" solimp=".9 .99 .003" solref=".015 1" material="body"/>
+      <geom type="capsule" condim="1" friction=".7" solimp=".9 .99 .003" solref=".003 1" material="body"/>
       <default class="thigh">
         <geom size=".06"/>
       </default>
@@ -116,6 +116,9 @@
         <geom name="head" type="sphere" size=".09"/>
         <camera name="egocentric" pos=".09 0 0" xyaxes="0 -1 0 .1 0 1" fovy="80"/>
       </body>
+      <body name="neck">
+        <geom type="cylinder" size=".03" fromto="0 0 .07 0 0 .1"/>
+      </body>
       <body name="waist_lower" pos="-.01 0 -.26">
         <geom name="waist_lower" fromto="0 -.06 0 0 .06 0" size=".06"/>
         <body name="pelvis" pos="0 0 -.165">

model/plugin/elasticity/poncho_flex.xml

@@ -30,11 +30,15 @@
     <global offwidth="800" offheight="800"/>
   </visual>
 
+  <default>
+    <geom solref="0.003 1"/>
+  </default>
+
   <worldbody>
     <light directional="false" diffuse=".2 .2 .2" specular="0 0 0" pos="0 0 5" dir="0 0 -1"/>
 
     <flexcomp name="f1" type="direct" rgba=".8 .2 .2 1" radius="0.01" dim="2" pos="0 0 2"
-              mass="3"
+              mass="1"
          point="0 0 0
                 -1 1 0
                 -0.875 1 0
@@ -1414,6 +1418,7 @@
                 398 399 418
                 398 376 378">
       <edge equality="true" damping="0.1"/>
+      <contact solref="0.003"/>
       <plugin plugin="mujoco.elasticity.shell">
         <config key="poisson" value="0"/>
         <config key="thickness" value="8e-3"/>