Improve documentation and add an example of training with half precision

[PabloCarmona]

Description and motivation

Related issue: #623

We need to improve the current docs related to half precision training and add an example to the examples folder related to it.

Proposed solution

• Create a new section inside official docs about how to use half-precision training.
• Add a new .py file with a code example on how to use half-precision training.

Alternatives and other information

• Create an interactive codelab that follows the docs and the example in the same place for the users to try it out faster. This is nice to have, but it is not mandatory to close the task.