[license] adjust example doc generation to new license headers

[license] create_cmakelists.py includes SPDX header

[license] add SPDX headers to Doxygen

[license] use CC-BY-4.0 for handbook documentation

[license] bulk license .md documentation with CC-BY-4.0

[license] use CC-BY-4.0 for example documentation

[license] bulk-license reference solutions

[license] use CC0-1.0 for other reference data

[license] bulk license .geo, .msh, figures, .pdf

[license] use CC-BY-4.0 for input files

[license] license modified reference data

[license] add header for original data source for embedded network example

[license] use CC-BY-4.0 for output of embedded example

The C preprocessor interprets the double backslash as
a newline and then warns because of possibly having an
unwanted multi-line comment. (False positive in this case.)

Credits to Martin Schneider for pointing out most of the things
that needed to be improved.