Added SPEC and VMEC high-level descriptions

[smiet]

Added documentation on how simsopt interfaces with the MHD codes SPEC and VMEC.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 91.98%. Comparing base (f0f0ec4) to head (d399e87). Report is 19 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #508      +/-   ##
==========================================
- Coverage   92.25%   91.98%   -0.28%     
==========================================
  Files          82       82              
  Lines       16015    16015              
==========================================
- Hits        14775    14731      -44     
- Misses       1240     1284      +44     

Flag	Coverage Δ
unittests	91.98% <ø> (-0.28%)	:arrow_down:

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[smiet]

@mishapadidar Just addressed all your points. Thanks for your time!

[mbkumar]

The generated doc is at https://simsopt.readthedocs.io/fixit_spec_documentation/mhd.html @smiet The document needs a revision. Please fix the VMEC and SPEC links so they are not explicit. Additionally check "simsopt.geo.SurfaceRZFourier`", which is hard to catch. Instead of saying "f90wrapped code is not re-entrant.", I'd say "f90wrap generates singleton classes". Please go through the docs one more time and fix small typos.

[smiet]

@mbkumar Should have addressed your issues now, sorry first time working with the .rst format.