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

python/mujoco/functions.cc

@@ -302,6 +302,7 @@ PYBIND11_MODULE(_functions, pymodule) {
         }
         return InterceptMjErrors(::mj_setState)(m, d, state.data(), spec);
       });
+  Def<traits::mj_setKeyframe>(pymodule);
   Def<traits::mj_addContact>(pymodule);
   Def<traits::mj_isPyramidal>(pymodule);
   Def<traits::mj_isSparse>(pymodule);