Add Android roadbook app guide

[lainollll]

Motivation

• Add a practical, buildable guide describing requirements and implementation approach for an Android roadbook app that imports GPX files and presents them as a roadbook-style sequence of steps for adventure motorbike use.

Description

• Add ROADBOOK_APP_GUIDE.md which documents the MVP feature set, suggested Kotlin/Jetpack Compose tech stack, GPX→roadbook conversion heuristics and example RoadbookStep model, UI layout and Compose component outline, location tracking recommendations, data flow, implementation milestones, and open questions for next steps.

Testing

• Documentation-only change, no automated tests were run.

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Codex Task