Document magic.json contract in README

[Copilot]

README’s magic.json section was a single high-level paragraph and did not describe the file contract. This PR adds a concise, schema-aligned spec so contributors know what belongs in magic.json and how to produce it reliably.

• What was missing

  • Clarifies magic.json purpose as Adafruit IO export metadata for built-in board components.
  • Explicitly links validation source: /boards/magic_schema.json.

• Schema contract added

  • Documents required top-level keys: exportVersion, exportedBy, exportedAt, exportedFromDevice, components.
  • Documents required exportedFromDevice keys: board, firmwareVersion.
  • Documents required per-component keys: name, type.
  • Notes that component objects must match one schema variant (oneOf: pin/I2C/PWM/pixel/DS18X20/UART/servo/display).

• Authoring guidance

  • Recommends configuring built-in components in Adafruit IO and exporting magic.json rather than hand-crafting.
  • Adds concrete example paths to existing magic.json files for reference.

• Example (added format guidance)

  | `exportedAt` | string | Timestamp when the file was exported (typically an ISO 8601 UTC timestamp from Adafruit IO, e.g. `2023-11-13T19:41:29.465Z`). |
  | `type` | string | WipperSnapper component type identifier (for example `push_button`, `dimmable_led`, `neopixel`, or `lc709203f:voltage`). |

[tyeth]

@brentru this is ready for review, and closes #241