board_spec: document the exposed connector conventions, connector types, accessories, the CLI and the editor

[ainyan03]

Summary

Documentation only (extras/board_spec/README.md, no code or catalog data changed).

• Exposed connectors: the conventions used when the connectors of every board were cataloged (Catalog exposed connectors and official references for M5 boards #346): write only values confirmed by official sources, port_a to port_e as the M5Unified pin-table contract, logical terminal types for undocumented layouts, multi_gpio, group, and what is not cataloged.
• Connector types and Accessories: what the files under connector_types/ and accessories/ hold (layout, positions, standards, rails, verified.layout; attaches, host_pins, settings) and how accessory wiring reaches the compositions and the M5Unified pin table.
• Directory map and command list: every directory that exists today, every cli/spec.js command with its output location, and what check verifies (formatting plus every generated output being present and current).
• Editor: moved out of the detection notes into its own section, with the page layout from Board spec editor: page list layout and readable connector pinouts #347.

node cli/spec.js check and npm test (236 tests) pass; the README is not an input of either.