[vcpkg] Improve Android doc (triplets, usage with cmake and prefab) (#11264)

* 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

1 files changed, 8 insertions, 0 deletions

diff --git a/docs/examples/vcpkg_android_example_cmake_script/my_lib.cpp b/docs/examples/vcpkg_android_example_cmake_script/my_lib.cpp
new file mode 100644
index 000000000..f0165d72d
--- /dev/null
+++ b/docs/examples/vcpkg_android_example_cmake_script/my_lib.cpp
@@ -0,0 +1,8 @@
+#include <json/json.h>
+
+int answer()
+{
+  Json::Value meaning_of;
+  meaning_of["everything"] = 42;
+  return meaning_of["everything"].asInt();
+}