* Android: add docs/examples/vcpkg_android_example_cmake/

* Add docs/users/android.md

* Improve Android prefab doc

Following additional informations given by @atkawa7 at
https://github.com/microsoft/vcpkg/pull/11264

* Link to android.md
* Update the prefab usage instructions: "vcpkg install"
    the 4 archs before exporting a prefab
* added --prefab-debug flag
* added gradle integration info
* reviewed the prefab output directory structure
  (from a dump of an actual export)

* docs/users/triplets.md: link to android.md

* docs/index.md: link to android.md

* android.md: specify 2 possibiities for android_ndk_home

* Added examples/vcpkg_android_example_cmake_script/cmake/vcpkg_android.cmake

Usage:

1. Main CMakeLists:
  if (VCPKG_TARGET_ANDROID)
      include("cmake/vcpkg_android.cmake")
  endif()

2. cmake invocation:
  cmake .. -DVCPKG_TARGET_ANDROID=ON -DANDROID_ABI=armeabi-v7a

* trigger pipeline build

* trigger pipelines