docs: add ADR-006 — hamburger menu for mobile navigation

Standardize on hamburger menu across all Code with Branko tutorials.
Closes #101.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: braboj

docs/decisions/006-mobile-navigation.md

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+# ADR-006: Hamburger menu for mobile navigation
+
+**Status:** Accepted
+**Date:** 2026-04-20
+
+## Context
+The Git tutorial (Astro site) needed a mobile navigation pattern. The Azure
+tutorial uses a hamburger menu; the Git tutorial initially used a horizontally
+scrollable tab bar. With 9 chapters, off-screen tabs were hard to discover.
+
+## Decision
+Use a hamburger menu for mobile navigation (breakpoint at 768 px), matching
+the Azure tutorial. This becomes the standard pattern across all Code with
+Branko tutorials.
+
+## Alternatives considered
+- **Horizontal scroll tabs**: Always visible, but items off-screen are not
+  discoverable — users don't know there are more chapters to the right
+- **Collapsible sidebar**: More common in documentation sites, but heavier
+  for a single-page tutorial layout
+
+## Consequences
+- Hamburger menu is the standard mobile nav for all tutorials
+- Future tutorials inherit this pattern
+- Navigation is hidden behind a tap but all 9 chapters are visible at once
+  when opened