Add Keras Website Documentation for Layers I (#2414)
Add Keras Website Documentation for Layers I (#2414)

* add core layers doc

* add advancedactivations and convolutional layers doc

* add dropout, embedding and normalization layers doc

* add pooling and recurrent layers doc

* update

* change

* update

* add merge

* add embedding

* change data

* modify

* update

* update

* change

* update

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