* 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

* [vcpkg docs] Add benchmarking 🏎 to the docs!

Also, minor changes to the testing docs.

* add documentation for the layout of the project

This PR does the following:

* fix tests -- now, they're always built in the CMake scripts, and they work on VS2015
  *add a new flag, BUILD_TESTING, which allows one to turn off testing builds
* Add documentation for running tests

* [maintainer-guide] Initial commit of maintainer guidelines

* [maintainer-guide] Rearrange sections

* [maintainer-guide] Add note about GitHub Draft PRs

* [maintainer-guide] Improvements

* [maintainer-guide] Address comments about versioning conventions

* [maintainer-guide] Add section about how portfiles interact with CMakeLists.txt

* [maintainer-guidelines] Add link from docs index

* Add detection for VCPKG_DOWNLOADS environment variable in vcpkgpaths.cpp.

* Pass the downloads directory from VcpkgPaths to cmake.

* Also fixup bootstrap on *nix.

* Make error message a little prettier.

* Make that bash script actually work :)

* [vcpkg] Alter Optional<> usage style

* [vcpkg-docs] Add section on Environment Variables to the docs

The current names and titles of the examples put too much emphasis on the package which is being demonstrated rather than the concept. This can be jarring for newcomers. This patch renames them to put the emphasis on the concept instead.