Feature: Adding user guide documentation

[fmalatino]

Description
This PR includes documentation as created by @xyuan. Upon approval a Read The Docs will be created for these documents.

How Has This Been Tested?
Does not affect functional code

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules
• New check tests, if applicable, are included

[FlorianDeconinck]

A few comments - this obviously needs much more work so let's just focus on fixing the basics and putting a structure in place