Create datum-concepts.md

[thomasdaly5]

Trying to provide an overview of Datum Concepts for new users.

[joshlreese]

Thanks for bootstrapping this document, it's a great start.

I've made a handful of recommendations and came across a couple spelling errors.

[joshlreese]

Styling request - We don't have a linter for this yet, but could we keep line length at 80 characters to help with reviewing content? It'd allow for more targeted comments and recommended changes.

If helpful, I use the Rewrap plugin in VSCode to help me with this - it allows me to select a block of text and wrap it at an arbitrary length. Another helpful plugin to use is the Markdown All in One plugin package.