Sphinx docs removed

- all Sphinx documentation files and scripts removed
- cmake Sphinx support removed
- some structural changes in tutorials
- legacy sample file inclusions replaced with `@snippet` command
- added PlantUML support (Doxygen 1.8.8 and later): pass `-DPLANTUML_JAR=<path-to-jar>` to cmake or set `PLANTUML_JAR` environment variable
- added html documentation customization files (header, footer, stylesheet)
- added Qt help generation support (pass `-DCMAKE_DOXYGEN_GENERATE_QHP=ON` to cmake)