YT-CPPGL-61: Prepare the GL module's documentation

[SpectraL519]

• Updated the project license
• Added the Doxyfile and mkdocs.yml configuration files and a Makefile with documentation targets
• Added documentation comments to the public API elements of the GL module
• Prepared the tutorial pages and images for the GL module's documenation
• Defined a utility script for generating documentation pages for the library's concepts
• Renamed the return_type and non_void_return_type and related utility to use the result naming
• Modified the PFS template implementation to accept arbitrary* search nodes
• Renamed the enqueue vertex predicate types and arguments for algorithm templates to use the (search) node naming
• Fixed the Dijkstra's algorithm implementation to properly utilize custom PFS search nodes and properly handle distance comparison within the min heap
• Removed unused/redundant elements

[SpectraL519]

ok