docs: fix README — devcontainer setup, submodule init, correct build presets

[Copilot]

README had a broken Ninja-Debug preset that doesn't exist, no mention of the cmake-conan submodule requirement, and no guidance on using the devcontainer.

Changes

• Devcontainer — new section documenting the bernedom/qtandroidbuilder-based devcontainer as the recommended setup path, with a link to the VS Code Dev Containers extension
• Submodule — callout block with --recurse-submodules clone command and git submodule update --init --recursive for existing clones
• Build presets — replaced the non-existent Ninja-Debug with the real preset names and matching build directories:

  # Desktop
  cmake --preset ci-ninja-desktop-debug
  cmake --build build
  
  # Android (single ABI)
  cmake --preset ci-ninja-android-arm64-v8a-debug
  cmake --build build_android_arm64-v8a
  

• Qt path — updated stale 6.6.1 reference in the multi-arch AAB section to 6.11.1, matching CMakePresets.json