Add docs for `User` functionality

[snowch]

The live docs can be found here: https://hpe-container-platform-community.github.io/hpecp-python-library/hpecp.html#

To view docs in gitpod - see here: https://github.com/hpe-container-platform-community/hpecp-python-library/blob/master/DEVELOPING.md#building-docs

This doc definition will need updating: https://raw.githubusercontent.com/hpe-container-platform-community/hpecp-python-library/master/docs/source/hpecp.rst

A new doc definition file will need to be created: hpecp.user.rst with the user documentation code. See here for an example: https://raw.githubusercontent.com/hpe-container-platform-community/hpecp-python-library/master/docs/source/hpecp.gateway.rst

[snowch]

@pramaku - this may be a good one for you to look at to understand the documentation process. Feel free to assign this to yourself.