Create plugin documentation

[alarshi]

This PR adds documentation on how to write a new plugin in GWB. It also fixes small typos that I came across while working on the write up.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (b72ce04) 93.49% compared to head (cd90f6b) 93.55%.
Report is 79 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #565      +/-   ##
==========================================
+ Coverage   93.49%   93.55%   +0.06%     
==========================================
  Files          92       92              
  Lines        6346     6346              
==========================================
+ Hits         5933     5937       +4     
+ Misses        413      409       -4     

see 4 files with indirect coverage changes

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[gassmoeller]

Very nice, just two small comments.

[MFraters]

Great, thanks for writing this up!

I think it might also be useful to add here that you will need to change the Class names in both the header and source files, and change the headerguard in the header file. Furthermore, I think it would be useful to add what information you need to put in the register function (e.g. ClassName, name used in the input file). Could you add something like this?

[alarshi]

Thank you for the review! I addressed your comments in the recent push. Let me know if we need more details.

[github-actions]

Benchmark	Main	Feature	Difference (99.9% CI)
Slab interpolation simple none	1.025 ± 0.003 (s=436)	1.025 ± 0.003 (s=444)	-0.0% .. +0.1%
Slab interpolation curved simple none	1.029 ± 0.004 (s=438)	1.029 ± 0.003 (s=439)	-0.1% .. +0.1%
Spherical slab interpolation simple none	1.262 ± 0.005 (s=338)	1.262 ± 0.003 (s=378)	-0.1% .. +0.0%
Slab interpolation simple curved CMS	1.099 ± 0.005 (s=458)	1.099 ± 0.006 (s=363)	-0.1% .. +0.1%
Spherical slab interpolation simple CMS	1.865 ± 0.004 (s=245)	1.862 ± 0.005 (s=241)	-0.2% .. -0.1%
Spherical fault interpolation simple none	1.275 ± 0.010 (s=334)	1.275 ± 0.004 (s=374)	-0.2% .. +0.1%
Cartesian min max surface	2.383 ± 0.022 (s=183)	2.381 ± 0.022 (s=197)	-0.4% .. +0.2%
Spherical min max surface	7.348 ± 0.018 (s=62)	7.351 ± 0.023 (s=63)	-0.1% .. +0.2%

[gassmoeller]

same as above

[alarshi]

Got it, thank you !