📝 add documentation and example for accept function

doc/examples/accept__string.cpp

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+#include <iostream>
+#include <iomanip>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+
+int main()
+{
+    // a valid JSON text
+    auto valid_text = R"(
+    {
+        "numbers": [1, 2, 3]
+    }
+    )";
+
+    // an invalid JSON text
+    auto invalid_text = R"(
+    {
+        "strings": ["extra", "comma", ]
+    }
+    )";
+
+    std::cout << std::boolalpha
+              << json::accept(valid_text) << ' '
+              << json::accept(invalid_text) << '\n';
+}

doc/examples/accept__string.link

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+<a target="_blank" href="https://wandbox.org/permlink/r5Tai1spihWHTdFA"><b>online</b></a>

doc/examples/accept__string.output

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+true false