[DOCFIX] Update tensorflow doc example
[DOCFIX] Update tensorflow doc example

Provide a new example in tensorflow documentation.

MNIST dataset is another well-known image recognition dataset that
classifies hand-writing digits.

The python script is slightly modified from
https://www.tensorflow.org/tutorials/quickstart/advanced.

The original second example uses `tf_cnn_benchmarsk.py`, which is more
suitable for tensorflow1 whereas the new example is using tensorflow2,
and is no longer under maintained. Therefore remove it.

pr-link: Alluxio/alluxio#14432
change-id: cid-5b7e70c133eb8417e0bab7d3396c72543572738e
Alluxio/alluxio,wwjiang007/alluxio,maobaolong/alluxio,Alluxio/alluxio,Alluxio/alluxio,wwjiang007/alluxio,Alluxio/alluxio,maobaolong/alluxio,Alluxio/alluxio,maobaolong/alluxio,wwjiang007/alluxio,Alluxio/alluxio,maobaolong/alluxio,wwjiang007/alluxio,maobaolong/alluxio,Alluxio/alluxio,wwjiang007/alluxio,wwjiang007/alluxio,wwjiang007/alluxio,wwjiang007/alluxio,Alluxio/alluxio,maobaolong/alluxio,wwjiang007/alluxio,maobaolong/alluxio,Alluxio/alluxio,maobaolong/alluxio,maobaolong/alluxio,maobaolong/alluxio,Alluxio/alluxio,wwjiang007/alluxio