impr: Improv config.flat and HyperBEAM configuration documentation

[speeddragon]

The configuration code has evolved, but this documentation was left behind. I've updated to represent the latest code in edge.

• HB_CONFIG_LOCATION is no longer used, being now HB_CONFIG.
• Update how to use flat structure, with examples with Map and List.

[samcamwilliams]

Thanks! But mind updating to point to the JSON configuration form as the primary type (HB_CONFIG value ending in a .json triggers that decoder)? It's much easier to write than a config.flat. Additionally, let's remove the part about 'more complex configurations require...' -- it isn't true anymore.