[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

4 files changed, 68 insertions, 0 deletions

diff --git a/docs/examples/vcpkg_android_example_cmake/.gitignore b/docs/examples/vcpkg_android_example_cmake/.gitignore
new file mode 100644
index 000000000..378eac25d
--- /dev/null
+++ b/docs/examples/vcpkg_android_example_cmake/.gitignore
@@ -0,0 +1 @@
+build
diff --git a/docs/examples/vcpkg_android_example_cmake/CMakeLists.txt b/docs/examples/vcpkg_android_example_cmake/CMakeLists.txt
new file mode 100644
index 000000000..7572bbbc8
--- /dev/null
+++ b/docs/examples/vcpkg_android_example_cmake/CMakeLists.txt
@@ -0,0 +1,5 @@
+cmake_minimum_required(VERSION 3.0)
+project(test)
+find_package(jsoncpp CONFIG REQUIRED)
+add_library(my_lib my_lib.cpp)
+target_link_libraries(my_lib jsoncpp_lib)
diff --git a/docs/examples/vcpkg_android_example_cmake/compile.sh b/docs/examples/vcpkg_android_example_cmake/compile.sh
new file mode 100755
index 000000000..1d1aa60a9
--- /dev/null
+++ b/docs/examples/vcpkg_android_example_cmake/compile.sh
@@ -0,0 +1,54 @@
+#
+# 1. Check the presence of required environment variables
+#
+if [ -z ${ANDROID_NDK_HOME+x} ]; then
+  echo "Please set ANDROID_NDK_HOME"
+  exit 1
+fi
+if [ -z ${VCPKG_ROOT+x} ]; then
+  echo "Please set VCPKG_ROOT"
+  exit 1
+fi
+
+#
+# 2. Set the path to the toolchains
+#
+vcpkg_toolchain_file=$VCPKG_ROOT/scripts/buildsystems/vcpkg.cmake
+android_toolchain_file=$ANDROID_NDK_HOME/build/cmake/android.toolchain.cmake
+
+
+#
+# 3. Select a pair "Android abi" / "vcpkg triplet"
+# Uncomment one of the four possibilities below
+#
+
+android_abi=armeabi-v7a
+vcpkg_target_triplet=arm-android
+
+# android_abi=x86
+# vcpkg_target_triplet=x86-android
+
+# android_abi=arm64-v8a
+# vcpkg_target_triplet=arm64-android
+
+# android_abi=x86_64
+# vcpkg_target_triplet=x64-android
+
+
+#
+# 4. Install the library via vcpkg
+#
+$VCPKG_ROOT/vcpkg install jsoncpp:$vcpkg_target_triplet
+
+#
+# 5. Test the build
+#
+rm -rf build
+mkdir build
+cd build 
+cmake .. \
+  -DCMAKE_TOOLCHAIN_FILE=$vcpkg_toolchain_file \
+  -DVCPKG_CHAINLOAD_TOOLCHAIN_FILE=$android_toolchain_file \
+  -DVCPKG_TARGET_TRIPLET=$vcpkg_target_triplet \
+  -DANDROID_ABI=$android_abi
+make
diff --git a/docs/examples/vcpkg_android_example_cmake/my_lib.cpp b/docs/examples/vcpkg_android_example_cmake/my_lib.cpp
new file mode 100644
index 000000000..f0165d72d
--- /dev/null
+++ b/docs/examples/vcpkg_android_example_cmake/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();
+}