feat: add examples

examples/boost_container/.gitignore

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+container

examples/boost_container/CMakeLists.txt

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+find_package(Boost 1.81.0)
+
+if(Boost_FOUND)
+    add_executable(container container.cpp)
+    target_link_libraries(container PRIVATE toml11::toml11 Boost::boost)
+    set_target_properties(container PROPERTIES
+        RUNTIME_OUTPUT_DIRECTORY "${CMAKE_CURRENT_SOURCE_DIR}")
+endif()

examples/boost_container/README.md

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+# container
+
+Use `boost::container::vector` and `boost::unordered_flat_map` as `array_type` and `table_type`.
+
+## build
+
+Install [boost](https://boost.org), especially after 1.81.0 that supports `unordered_flat_map`.
+
+Then, build toml11 with `-DTOML11_BUILD_EXAMPLES=ON`
+
+```cpp
+$ cmake -B ./build/ -DTOML11_BUILD_EXAMPLES=ON
+```

examples/boost_container/container.cpp

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+#include <boost/container/small_vector.hpp>
+#include <boost/unordered/unordered_flat_map.hpp>
+
+#include <toml.hpp>
+
+#include <iostream>
+
+struct boost_config
+{
+    using comment_type  = toml::preserve_comments;
+
+    using boolean_type  = bool;
+    using integer_type  = std::int64_t;
+    using floating_type = double;
+    using string_type   = std::string;
+
+    template<typename T>
+    using array_type = boost::container::small_vector<T, 8>;
+    template<typename K, typename T>
+    using table_type = boost::unordered_flat_map<K, T>;
+
+    static toml::result<integer_type, toml::error_info>
+    parse_int(const std::string& str, const toml::source_location src, const std::uint8_t base)
+    {
+        return toml::read_int<integer_type>(str, src, base);
+    }
+    static toml::result<floating_type, toml::error_info>
+    parse_float(const std::string& str, const toml::source_location src, const bool is_hex)
+    {
+        return toml::read_float<floating_type>(str, src, is_hex);
+    }
+};
+
+int main(int argc, char** argv)
+{
+    if(argc != 2)
+    {
+        std::cerr << "usage: ./container <input.toml>" << std::endl;
+        return 1;
+    }
+
+    const auto input = toml::parse_str<boost_config>(argv[1]);
+
+    std::cout << toml::format(input) << std::endl;
+
+    return 0;
+}