Add Evaluate Usage for Keras and Tensorflow (#370)

* Added Keras example for Evaluate metrics

Created a markdown file to show how to use Evaluate
when training and evaluate a model using Keras and Tensorflow.

* Added example for using Evaluate with Keras (#325)

Second commit for adding issue number.

* Apply fixes from code review

Fixes included some rephrasing, changing to use keras logs instead of print, and focusing on evaluating on the test set.

Co-authored-by: Leandro von Werra <lvwerra@users.noreply.github.com>

* Added extra sentence to clarify rounding and fixed typo

Typo from last commit accidently broke a code box.

* Added ref in toctree and accuracy in Keras Docs

Fixed incorrect ref addition to toctree, and added a
comment describing the acc score of the example
in the keras documentation.

Co-authored-by: Leandro von Werra <lvwerra@users.noreply.github.com>