Add proper C++20 module support (#4799)

docs/mkdocs/docs/features/modules.md

@@ -0,0 +1,33 @@
+# Modules
+
+This library has experimental support for C++ modules, introduced in C++20. The library can be imported by writing `import nlohmann.json;` instead of `#include <nlohmann/json.hpp>`. 
+
+Please be aware that the module is experimental and a full test is outstanding, and the exported symbols are subject to change.
+
+## Requirements
+The `nlohmann.json` module requires that the build system is configured to build and resolve modules when imported. Obviously, as modules were introduced in C++20, this feature can only be used in C++20 and subsequent versions.
+
+To enable building the `nlohmann.json` module (which is not done by default), the macro `NLOHMANN_JSON_BUILD_MODULES` must be passed to the build system.
+
+## Example
+When using modules rather than headers, the previous example for creating a `json` object through a JSON file, would instead be:
+```cpp
+import std;
+import nlohmann.json;
+
+using json = nlohmann::json;
+
+// ...
+
+std::ifstream f("example.json");
+json data = json::parse(f);
+```
+
+## Modules do not export macros
+It should be noted that as modules do not export macros, the `nlohmann.json` module will not export any macros, but rather only the following symbols:
+- `nlohmann::adl_serializer`
+- `nlohmann::basic_json`
+- `nlohmann::json`
+- `nlohmann::json_pointer`
+- `nlohmann::ordered_map`
+- `nlohmann::ordered_json`

docs/mkdocs/mkdocs.yml

@@ -77,6 +77,7 @@ nav:
       - features/json_pointer.md
       - features/json_patch.md
       - features/merge_patch.md
+      - features/modules.md
       - 'nlohmann Namespace': features/namespace.md
       - features/object_order.md
       - Parsing: