#179 Migrate the gh-page content with MkDocs

[fktn-k]

Migration have been completed from Doxygen to MkDocs to publish the documentation on the gh-pages.
From now on, every commit on the develop branch which includes changes in documentation files will update the contents on gh-pages.
Furthermore, dependencies on Doxygen have been removed except for the comment style in the library source files.

[coveralls]

coverage: 100.0%. remained the same
when pulling 215122d on feature/179_migrate_ghpage_content_with_mkdocs
into 3068884 on develop.