Documentation versions tested overnight

[albert-github]

In the overnight documentation overview (https://cgal.geometryfactory.com/CGAL/Manual_doxygen_test/) we see that there are 3 doxygen versions used:

• 1.8.13 (released December 29, 2016)
• 1.9.6 (released December 27, 2022)
• master

The 1.9.6 version is (as of CGAL 5.6) used in the production environment. The 1.8.13 version is, to the best of my knowledge, used in "XML-mode" to see whether or not here are regressions in the documentation.

The current doxygen version is 1.15.0

In my opinion it would be wise to abandon in the overnight builds) the 1.8.13 version, make the 1.9.6 the version for comparison and doxygen 1.15.0 ready for future production so:

• 1.9.6 (for production)
• 1.15.0 (for future production)
• master