[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