DOC: Added TensorFlow docs

Description of TensorFlow modules available at IT4Innovations.

docs.it4i/anselm/software/machine-learning/introduction.md

+# Machine Learning
+
+This section overviews machine learning frameworks and libraries available on the Anselm cluster.
+
+## TensorFlow
+
+Load TensorFlow module:
+
+```console
+$ ml Tensorflow
+```
+
+Test module:
+
+```console
+$ ml Tensorflow
+```
+
+Read more about available versions at the [TensorFlow page](tensorflow).