Add Parameter Documentation for Subducting Plate Feature

[danieldouglas92]

Update the manual by adding the section for the subducting plate feature in parameter documentation, addresses a point in Issue #766.

[github-actions]

Benchmark	Main	Feature	Difference (99.9% CI)
Slab interpolation simple none	1.213 ± 0.015 (s=349)	1.213 ± 0.012 (s=395)	-0.3% .. +0.2%
Slab interpolation curved simple none	1.217 ± 0.010 (s=393)	1.216 ± 0.008 (s=349)	-0.2% .. +0.1%
Spherical slab interpolation simple none	1.224 ± 0.010 (s=371)	1.224 ± 0.012 (s=367)	-0.2% .. +0.2%
Slab interpolation simple curved CMS	1.254 ± 0.014 (s=373)	1.255 ± 0.023 (s=347)	-0.3% .. +0.5%
Spherical slab interpolation simple CMS	1.635 ± 0.015 (s=288)	1.638 ± 0.013 (s=264)	-0.1% .. +0.4%
Spherical fault interpolation simple none	1.234 ± 0.010 (s=357)	1.238 ± 0.010 (s=373)	+0.1% .. +0.5%
Cartesian min max surface	2.729 ± 0.020 (s=169)	2.731 ± 0.020 (s=163)	-0.2% .. +0.3%
Spherical min max surface	7.758 ± 0.068 (s=59)	7.776 ± 0.198 (s=59)	-1.0% .. +1.4%

[MFraters]

Thanks for writing this up, very nice! Also cool to see how you made the figures to explain the concepts. I have a few small comments, but otherwise looks good to me.

1. I guess you are trying to get into math mode with with the $ around some variables, but it doesn't render correctly. Maybe we to enable an extra plugin in sphinx?
2. Currently there is a limitation that the number of segements need to be the same in all sections. Could you add that?
3. In the downdip_sections.png figure, the black cross-sections are a bit hard to see. Maybe a lighter color or an outline would help.

[danieldouglas92]

@MFraters Thanks for the quick review! I addressed points 1 and 2, I just made M and N italicized, I don't think we need to worry about the math mode extension for sphinx at least for this PR. For point 3 I'll remake the figures later tonight, I also think the font should be a little larger, so I will comment at you again when it's ready for you take another look.