Add CMake fetchcontent documentation and tests

Github issue:
https://github.com/nlohmann/json/issues/2073

nlohmann::json documents 2 way of depending on it using CMake
1) Copy-paste the project/source into your own project.
2) Install nlohman::json and then use find_package.

(1) pollutes your git repository, (2) requires everyone to install the
dependencies themselves.

Since 2018, CMake provide some kind of 'package manager' features using
[FetchContent](https://cmake.org/cmake/help/v3.17/module/FetchContent.html)
It gives the following:
~~~cmake
include(FetchContent)

FetchContent_Declare(json
  GIT_REPOSITORY https://github.com/nlohmann/json
  GIT_TAG v3.7.3)

FetchContent_GetProperties(json)
if(NOT json_POPULATED)
  FetchContent_Populate(json)
  add_subdirectory( ${json_SOURCE_DIR} ${json_BINARY_DIR} EXCLUDE_FROM_ALL)
endif()
~~~

Then declares the dependency in the target using it:
~~~cmake
target_link_library(my_project PRIVATE nlohmann_json::nlohmann_json
~~~

This patch updates the documentation and provides tests.

test/cmake_fetch_content/CMakeLists.txt

@@ -0,0 +1,18 @@
+if (${CMAKE_VERSION} VERSION_GREATER "3.11.0")
+  add_test(NAME cmake_fetch_content_configure
+    COMMAND ${CMAKE_COMMAND}
+    -G "${CMAKE_GENERATOR}"
+    -DCMAKE_CXX_COMPILER=${CMAKE_CXX_COMPILER}
+    -Dnlohmann_json_source=${PROJECT_SOURCE_DIR}
+    ${CMAKE_CURRENT_SOURCE_DIR}/project
+  )
+  add_test(NAME cmake_fetch_content_build
+    COMMAND ${CMAKE_COMMAND} --build .
+  )
+  set_tests_properties(cmake_fetch_content_configure PROPERTIES
+    FIXTURES_SETUP cmake_fetch_content
+  )
+  set_tests_properties(cmake_fetch_content_build PROPERTIES
+    FIXTURES_REQUIRED cmake_fetch_content
+  )
+endif()