* 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
Use DOCTEST_* macros in place of predefined compiler macros for compiler
detection and version checks.
The suppression of warning -Wrange-loop-construct in unit-items.cpp
causes GCC<11 to warn about pragmas. Suppressed by adding a version
check.