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

docs/mkdocs/docs/api/operator_literal_json.md

@@ -0,0 +1,59 @@
+# <small>nlohmann::</small>operator""_json
+
+```cpp
+json operator "" _json(const char* s, std::size_t n);
+```
+
+This operator implements a user-defined string literal for JSON objects. It can be used by adding `#!cpp _json` to a
+string literal and returns a [`json`](json.md) object if no parse error occurred.
+
+Use any of the following lines to bring the operator into scope:
+```cpp
+using namespace nlohmann::literals;
+using namespace nlohmann::json_literals;
+using namespace nlohmann::literals::json_literals;
+using namespace nlohmann;
+```
+
+Alternatively, define [`JSON_USE_GLOBAL_UDLS`](macros/json_use_global_udls.md) to make them available in the global
+namespace.
+## Parameters
+
+`s` (in)
+:   a string representation of a JSON object
+
+`n` (in)
+:   length of string `s`
+
+## Return value
+
+[`json`](json.md) value parsed from `s`
+
+## Exceptions
+
+The function can throw anything that [`parse(s, s+n)`](basic_json/parse.md) would throw.
+
+## Complexity
+
+Linear.
+
+## Examples
+
+??? example
+
+    The following code shows how to create JSON values from string literals.
+     
+    ```cpp
+    --8<-- "examples/operator_literal_json.cpp"
+    ```
+    
+    Output:
+    
+    ```json
+    --8<-- "examples/operator_literal_json.output"
+    ```
+
+## Version history
+
+- Added in version 1.0.0.
+- Moved to namespace `nlohmann::literals::json_literals` in 3.11.0.