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-element_access2.cpp

@@ -10,6 +10,7 @@
 #include "doctest_compatibility.h"
 
 #include <nlohmann/json.hpp>
+using namespace nlohmann::literals; // NOLINT(google-build-using-namespace)
 
 TEST_CASE_TEMPLATE("element access 2", Json, nlohmann::json, nlohmann::ordered_json)
 {