Added Equella table structures to the guides directory and linked the docs to the Reporting Guide

[matjmiles]

No description provided.

[cbeach47]

@matjmiles - sry for the long delay in a review.

With the difficultly of updating PDFs in github, and the recent advent of the openEQUELLA Community Repository, can you instead add these PDFs into that repository, and open an issue ticket (or create another PR) to list out the various EVENT_TYPE / EVENT_CATEGORY combinations in this documentation?

For the schema, there is a older, but still relevant schema diagram for all openEQUELLA tables here: https://github.com/equella/equella.github.io/blob/master/guides/equella_schema.zip that could be added to the ReportingGuide page (or highlighted).