Clean up and document project files (#4560)

docs/mkdocs/docs/integration/xmake/example.cpp

@@ -0,0 +1,10 @@
+#include <nlohmann/json.hpp>
+#include <iostream>
+#include <iomanip>
+
+using json = nlohmann::json;
+
+int main()
+{
+    std::cout << std::setw(4) << json::meta() << std::endl;
+}

docs/mkdocs/docs/integration/xmake/xmake.lua

@@ -0,0 +1,8 @@
+add_requires("nlohmann_json")
+
+add_rules("mode.debug", "mode.release")
+target("xm")
+    set_kind("binary")
+    add_files("example.cpp")
+    add_packages("nlohmann_json")
+    set_languages("cxx11")