Map document structure to project brief

The original project brief for the GTR project anticipated a number of deliverables and proposed exit criteria for them. These are still remarkably relevant but there are some additions and improvements that have been made in more than a year of work by the team.

We need to update the explanation of the document arrangement and mapping to the original project brief to show how what we've done meets (or possibly even exceeds) the expectations of the brief.

The "information architecture" is described by the readme.md file here: https://opensource.unicc.org/un/unece/uncefact/gtr/-/blob/main/docs/Documents/readme.md

And the contents of this file are rendered as a web page here: https://un.opensource.unicc.org/unece/uncefact/gtr/docs/Documents/

We need to make changes to the readme.md file to provide a better explanation of the documents.