add doc for gelu into index

add gelu into document index (for both english and chinese doc)

doc/fluid/api/layers.rst

@@ -113,6 +113,7 @@ fluid.layers
     layers/gather_tree.rst
     layers/gaussian_random.rst
     layers/gaussian_random_batch_size_like.rst
+    layers/gelu.rst
     layers/generate_mask_labels.rst
     layers/generate_proposal_labels.rst
     layers/generate_proposals.rst

doc/fluid/api_cn/layers_cn.rst

@@ -118,6 +118,7 @@ fluid.layers
     layers_cn/gather_tree_cn.rst
     layers_cn/gaussian_random_batch_size_like_cn.rst
     layers_cn/gaussian_random_cn.rst
+    layers_cn/gelu_cn.rst
     layers_cn/generate_mask_labels_cn.rst
     layers_cn/generate_proposal_labels_cn.rst
     layers_cn/generate_proposals_cn.rst