wiki: add Architecture page; Xbox auth flow on Feather; nav updates - Architecture page (moved from the repo), links rewritten for the wiki. - Feather: lead with Xbox authentication (plug real controller until LED, then swap for drums); LED = auth-complete indicator. - Home/sidebar: Architecture under 'How it works'; CONTRIBUTING in dev reference.
Rewrite wiki: Home, Feather bring-up, Hi-Hat, Logging, Design History Cohesive doc set describing the current repo state (USB+serial MIDI inputs, guitar WIP, WLA multi-instrument architecture, FreeRTOS-SMP). Replaces the seeded stub.