Support serializing multiple YAML docs

[fktn-k]

This PR has added the new support of serializing multiple YAML documents by implementing the fkyaml::basic_node::serialize_docs() function.
It accepts a vector of basic_node objects like the fkyaml::basic_node::deserialize_docs() function returns, and returns a string as the serialization result with separation lines (...) between documents.
See the updated API documentation for details about the function.

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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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[coveralls]

Pull Request Test Coverage Report for Build 9532608170

Details

• 15 of 15 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 9528914558:	0.0%
Covered Lines:	3928
Relevant Lines:	3928

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