fkYAML version 0.3.9

* added workflow job for intel icpx compiler

* add compile options for IntelLLVM compilers for testing

* added IntelLLVM compiler to the supported compilers list

* cleanup the workflow jobs for IntelLLVM compiler

* support parsing multiple YAML documents

* added documentation for the deserialize_docs() function

* deleted useless comment-outs in mkdocs.yml

* added test cases for invalid end-of-documents markers

* support serialization of multiple YAML documents

* reorganized test cases for deserialization/serialization

* add documentation for the serialize_docs() function

* ignore misjudged uncovered line in the serializer

* allow trailing comma in flow mapping/sequence

* fixed memory leak in deserializer error cases

* added missing doxygen comments in the deserializer source

* fixed missing file closing in test suite

* removed ci jobs which used to run with the macOS11 runner image (not working anymore)

* updated the supported compilers list in docs

* fixed lacked line advancement for block mappings as a block sequence entry

* run amalgamation

* cleaned up redundant impl for node traversals before parsing a mapping key node

* modified comments for parsing node props

* apply node properties for the root node

* support node properties for the root node in the serialization feature

* updated docs for serialization

* update configs for reuse v4

* updated Jinja2 version to 3.1.4 to fix a Codacy's security issue