docs: in state vector notebook, add link to API reference

docs/explanations/state-vectors-and-gates.ipynb

@@ -159,7 +159,7 @@
    "source": [
     "## Gates\n",
     "\n",
-    "In ffsim, you apply a unitary gate to a state vector by calling a function whose name begins with `apply_`. For example, the function for applying an orbital rotation is called `apply_orbital_rotation`. The first argument to the function is always the state vector itself. The number of orbitals, as well as the number of alpha and beta electrons, are passed as the arguments `norb` and `nelec`.\n",
+    "In ffsim, you apply a unitary gate to a state vector by calling a function whose name begins with `apply_`. For example, the function for applying an orbital rotation is called `apply_orbital_rotation`. The first argument to the function is always the state vector itself. The number of orbitals, as well as the number of alpha and beta electrons, are passed as the arguments `norb` and `nelec`. See the [API reference](../api/ffsim.rst) for the full list of supported gates and their definitions (search for `ffsim.apply_`).\n",
     "\n",
     "The following code cell generates a random orbital rotation (represented by an $N \\times N$ unitary matrix) and applies it to the Hartree-Fock state vector we created previously."
    ]