Add <damper> actuator shortcut and related mjGAIN_AFFINE.

**Rationale**

A general `mjGAIN_AFFINE` actuator adds the term `force = (c + kp * length + kv * velocity) * ctrl`. The damper shortcut restricts it to `kv * velocity * ctrl` and sets the `ctrllimited` attribute to true.

**API**

```xml
<actuator>
  <damper ctrlrange="0 1" kv="1"/>
</actuator>
```

* Required attributes: `ctrlrange` (>=0)
* Optional attributes: `kv` (>=0)

PiperOrigin-RevId: 455366866
Change-Id: I88773abbeccabb6442f215c9fc05e205df63f692

doc/APIreference.rst

@@ -403,6 +403,7 @@ mjtGain
    typedef enum _mjtGain
    {
        mjGAIN_FIXED        = 0,        // fixed gain
+       mjGAIN_AFFINE,                  // const + kp*length + kv*velocity
        mjGAIN_MUSCLE,                  // muscle FLV curve computed by mju_muscleGain()
        mjGAIN_USER                     // user-defined gain type
    } mjtGain;