Fix CMake documentation to prevent GoogleTest install

[ypujante]

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I added GoogleTest to my project by following the Setup section of the Building with CMake documentation page in order to add unit tests. The issue is that the example does NOT disable installing GoogleTest. As a result I ended up polluting my system with headers and libs from GoogleTest when it is clearly not what I wanted to do...

Here is a proposed fix:

cmake_minimum_required(VERSION 3.14)
project(my_project)

# GoogleTest requires at least C++11
set(CMAKE_CXX_STANDARD 11)

include(FetchContent)
FetchContent_Declare(
  googletest
  URL https://github.com/google/googletest/archive/609281088cfefc76f9d0ce82e1ff6c30cc3591e5.zip
)
# For Windows: Prevent overriding the parent project's compiler/linker settings
set(gtest_force_shared_crt ON CACHE BOOL "" FORCE)

# So that GoogleTest does not get installed with this project
option(INSTALL_GTEST "Enable installation of googletest." OFF)

FetchContent_MakeAvailable(googletest)