WIP draft of numbers concept

[colinleach]

This is an unfinished draft, NOT ready for formal review.

Known limitations:

• introduction.md and links.json are both empty
• References within about.md are very Wikipedia-heavy. I need to find more Julia-specific links.
• We need to decide what exercise to pair this with, before writing introduction.md

I'm pushing this to concepts.wip to get it up on GH for discussion. Also as an indication of the way I'm thinking (rightly or wrongly) about the syllabus. Also as I knind of apology for pushing back quite aggressively against adding more content to basics - I wanted to discuss things properly in this separate concept.