[DOC] Reintegration of website sources into main source
- part of migration towards multi-version website
- note: 2 reversions here:
- 'VERSION' file is back, read both by CMakeLists.txt for project() command and also by script piloting Doxygen to generate dox. Changing VERSION will change both in sync.
- release_notes.md moves back to the reinstated doc/doxygen directory