Add to_node() implementations for STL container types

[fktn-k]

This PR has added builtin implementations of the to_node() function which are used when constructing a fkyaml::basic_node object with arbitrary types.
Before this PR, minimal builtin implementation was provided so that converting to a basic_node object from its template parameter types (like fkyaml::basic_node::sequence_type), which wasn't much helpful.
Now, objects of many existing STL container types (like std::vector, std::unordered_map or std::tuple) can be used as an argument of the fkyaml::basic_node constructor.
See the updated documentation for supported STL container types.

Furthermore, you can, of course, provide your own custom to_node() implementation for non-standard types just as before and such a type can be used as the element type of STL container types.

If you run the following code, for example:

// define a type and to_node implementation for it
namespace ns {
struct novel {
    std::string title;
    int price;
    int published_at;
};

template <typename BasicNodeType>
void to_node(BasicNodeType& node, const novel& novel) {
   // convert the `novel` to a mapping.
   node = {{"title", novel.title}, {"price", novel.price}, {"published_at", novel.published_at}};
}
} // namespace ns

// use them in the code
// fkYAML provides builtin to_node() implementation for std::list<T>
// to_node() for the nested `ns::novel` as T will be recursively executed in the constructor.
std::list<ns::novel> novels = {
    {"foo", 100, 2023}, {"bar", 120, 2024}, {"baz", 80, 2025}
};
fkyaml::node n = novels;

// print the node value
std::cout << n << std::endl;

then the output will be:

-
  price: 120
  published_at: 2024
  title: bar
-
  price: 80
  published_at: 2025
  title: baz
-
  price: 100
  published_at: 2023
  title: foo

---

Pull Request Checklist

Read the CONTRIBUTING.md file for detailed information.

• Changes are described in the pull request or in a referenced issue.
• The test suite compiles and runs without any error.
• The code coverage on your branch is 100%.
• The documentation is updated if you added/changed a feature.

Please don't

• The C++11 support varies between different compilers and versions. Please note the list of supported compilers. Some compilers like GCC 4.7 (and earlier), Clang 3.3 (and earlier), or Microsoft Visual Studio 13.0 and earlier are known not to work due to missing or incomplete C++11 support. Please refrain from proposing changes that work around these compiler's limitations with #ifdefs or other means.
• Please refrain from proposing changes that would break YAML specifications. If you propose a conformant extension of YAML to be supported by the library, please motivate this extension.
• Please do not open pull requests that address multiple issues.

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