Add diagram for HELICS Terminology page in User Guide

[trevorhardy]

This wall of text could use a diagram or two to help make the ideas more digestible.

[nightlark]

Oh, that really is a wall of text.

Some observations:

• each bullet point/term has a full paragraph, most terms feel like the core definition appears in the first 1-2 sentences, but then crams in extra details that makes it harder to read in part due to...
• bulleted list packs all the text tightly together

I think giving each term its own sub-heading with the 1-2 sentence "definition" and some formatting for the extra details would be easier on the eyes, and make it much more readable.

[bpalmintier]

@trevorhardy: Your 2020 IEEE Task Force presentation (and likely others, e.g. tutorial) may have some good starting point figures.) Just came across them building another presentation.

[trevorhardy]

Fixed in commit 58d33c519.