Clean up and document project files (#4560)

docs/mkdocs/docs/examples/error_handler_t.cpp

@@ -0,0 +1,24 @@
+#include <iostream>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+
+int main()
+{
+    // create JSON value with invalid UTF-8 byte sequence
+    json j_invalid = "ä\xA9ü";
+    try
+    {
+        std::cout << j_invalid.dump() << std::endl;
+    }
+    catch (const json::type_error& e)
+    {
+        std::cout << e.what() << std::endl;
+    }
+
+    std::cout << "string with replaced invalid characters: "
+              << j_invalid.dump(-1, ' ', false, json::error_handler_t::replace)
+              << "\nstring with ignored invalid characters: "
+              << j_invalid.dump(-1, ' ', false, json::error_handler_t::ignore)
+              << '\n';
+}