Add mj_setKeyframe function to save current state in k-th model keyframe.

Fixes #1719.

Also improve documentation of `mju_sigmoid`, was previously only documented in changelog.

PiperOrigin-RevId: 642637458
Change-Id: I811a0bf8f881c8b76c9a62d2faec8fdec4e7585b

src/engine/engine_support.h

@@ -43,6 +43,8 @@ MJAPI void mj_getState(const mjModel* m, const mjData* d, mjtNum* state, unsigne
 // set state
 MJAPI void mj_setState(const mjModel* m, mjData* d, const mjtNum* state, unsigned int spec);
 
+// copy current state to the k-th model keyframe
+MJAPI void mj_setKeyframe(mjModel* m, const mjData* d, int k);
 
 //-------------------------- sparse chains ---------------------------------------------------------