README: more restructuring, to emphasize involvement/participation

[ddbeck]

trafficstars

This completes the bulk of the remaining work described in #14. Basically, it's structured to drive people into the docs that are relevant to them. The biggest single change here is to move the introduction wall of text into a separate file. Hopefully, this overall structure will pique interest with the abbreviated pitch, which will motivate readers into the more detailed docs (e.g., the intro or white paper) or into the Discord.

A somewhat easier to look at preview: https://github.com/ddbeck/nomia/tree/14-README-refresh-more#readme