Replace getters for reference to node values

[fktn-k]

This PR added the following new functions to replace the existing get_value_ref() function. See reference pages for each of them for detailed information and example usages.

• as_seq()
• as_map()
• as_bool()
• as_int()
• as_float()
• as_str()

Also, get_value_ref() has been deprecated and will be removed in a future update.
See migration guide in the reference page for get_value_ref().

The reasons for the deprecation:

• too verbose interface
  Although this function seems to take arbitrary types for reference, the accepted types are actually only (const) sequence_type&, (const) mapping_type&, (const) boolean_type&, (const) integer_type&, (const) float_number_type& and (const) string_type&.
• not easy to understand until a library user gets used to the interface.
  Due to explicit reference type specification, the resulting code becomes unnecessarily long and seemingly complex in contrast to the internal implementation, i.e., just returning a reference to an appropriate type.

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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.

[github-actions]

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