Move UDLs out of the global namespace (#3605)

* Move UDLs into nlohmann::literals::json_literals namespace

* Add 'using namespace' to unit tests

* Add 'using namespace' to examples

* Add 'using namespace' to README

* Move UDL mkdocs pages out of basic_json/

* Update documentation

* Update docset index

* Add JSON_GlobalUDLs CMake option

* Add unit test

* Build examples without global UDLs

* Add CI target

tests/src/unit-json_pointer.cpp

@@ -11,6 +11,7 @@
 #define JSON_TESTS_PRIVATE
 #include <nlohmann/json.hpp>
 using nlohmann::json;
+using namespace nlohmann::literals; // NOLINT(google-build-using-namespace)
 
 #include <sstream>