(Fix-It Week) Updated the documentation of the `to_mgrid` function

[mishapadidar]

Summary This PR updates the documentation of the to_mgrid method of MagneticField objects. The changes to the documentation describe how the mgrid file is used by free boundary VMEC, and how to set the arguments of the function. The discuss is largely taken from the vmec wiki.

Screenshots

Tests No unit tests needed.

[mishapadidar]

Updated documentation and uploaded new screenshot

[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 (5e6b02c). Report is 155 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #503      +/-   ##
==========================================
- 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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[mishapadidar]

Updated documentation to reflect the capability with respect to the number of current groups.

Screenshot of to_mgrid doc Screenshots of MGrid doc