Add developer documentation for astyle

[danieldouglas92]

Add details for downloading, installing, and using astyle for worldbuilder.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (40655ff) 93.46% compared to head (98f5bee) 93.49%.
Report is 13 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #541      +/-   ##
==========================================
+ Coverage   93.46%   93.49%   +0.03%     
==========================================
  Files          92       92              
  Lines        6301     6346      +45     
==========================================
+ Hits         5889     5933      +44     
- Misses        412      413       +1     

see 1 file with indirect coverage changes

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[MFraters]

Great, thanks for writing this up! The aspect wiki also has a oneline version of the to compile and install, could you also add there here and squash your commits?

https://github.com/geodynamics/aspect/wiki/Indenting-code-and-installing-the-correct-version-of-astyle

Otherwise looks good to go.

[github-actions]

Benchmark	Main	Feature	Difference (99.9% CI)
Slab interpolation simple none	1.024 ± 0.007 (s=449)	1.024 ± 0.007 (s=433)	-0.1% .. +0.2%
Slab interpolation curved simple none	1.024 ± 0.004 (s=414)	1.025 ± 0.006 (s=467)	-0.0% .. +0.2%
Spherical slab interpolation simple none	1.262 ± 0.005 (s=355)	1.262 ± 0.004 (s=361)	-0.1% .. +0.1%
Slab interpolation simple curved CMS	1.097 ± 0.004 (s=417)	1.097 ± 0.004 (s=406)	-0.1% .. +0.1%
Spherical slab interpolation simple CMS	1.865 ± 0.007 (s=239)	1.864 ± 0.009 (s=246)	-0.2% .. +0.0%
Spherical fault interpolation simple none	1.271 ± 0.003 (s=378)	1.272 ± 0.004 (s=332)	-0.0% .. +0.1%
Cartesian min max surface	2.367 ± 0.019 (s=209)	2.369 ± 0.023 (s=174)	-0.2% .. +0.4%
Spherical min max surface	7.324 ± 0.015 (s=61)	7.325 ± 0.028 (s=64)	-0.2% .. +0.2%

[MFraters]

I have one small comment on the new text, otherwise good to go.