Add documentation for s3 usage with TensorFlow (#16923)

* Add documentation for s3 usage with TensorFlow

This fix adds a very priliminary documentation for s3 usage
with TensorFlow, as an attempt to address the comment 11089.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Add s3 documentation to index page for deployment.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Add left navigation link for s3 docs.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Change example to use tf.data instead of old input style

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>