Update doxygen design

[gassmoeller]

This is an update to the doxygen html stylesheet similar to geodynamics/aspect#6094. The new design is more modern, but also in particular scales better to mobile devices and other resolutions, and supports a switch between light and dark theme.

Old design:

New design:

[github-actions]

Benchmark	Main	Feature	Difference (99.9% CI)
Slab interpolation simple none	1.117 ± 0.015 (s=389)	1.116 ± 0.011 (s=420)	-0.4% .. +0.2%
Slab interpolation curved simple none	1.117 ± 0.010 (s=407)	1.117 ± 0.011 (s=401)	-0.2% .. +0.2%
Spherical slab interpolation simple none	1.093 ± 0.008 (s=389)	1.094 ± 0.010 (s=436)	-0.1% .. +0.2%
Slab interpolation simple curved CMS	1.164 ± 0.010 (s=375)	1.168 ± 0.009 (s=399)	+0.1% .. +0.5%
Spherical slab interpolation simple CMS	1.446 ± 0.011 (s=318)	1.447 ± 0.012 (s=307)	-0.1% .. +0.3%
Spherical fault interpolation simple none	1.100 ± 0.008 (s=388)	1.102 ± 0.010 (s=432)	-0.0% .. +0.3%
Cartesian min max surface	2.542 ± 0.016 (s=185)	2.543 ± 0.014 (s=171)	-0.2% .. +0.2%
Spherical min max surface	7.215 ± 0.180 (s=56)	7.199 ± 0.066 (s=71)	-1.4% .. +1.0%

[MFraters]

I forgot to ask, can you make a changlog entry (and a hackathon entry)?