Update API docs

[f0uriest]

• Adds some new API documentation grouping together common functionality by topic with brief explanations, rather than just a big alphabetical list (the alphabetical listing is still kept for reference)
• Also updates the "quick start" section of the readme to focus more on the tutorials and python API rather than the command line interface.
• Misc other formatting fixes to notebooks to get all the headings to play nice

Resolves #907

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.71%. Comparing base (cf8ff2c) to head (6675f44).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #920   +/-   ##
=======================================
  Coverage   94.71%   94.71%           
=======================================
  Files          86       86           
  Lines       21207    21207           
=======================================
  Hits        20086    20086           
  Misses       1121     1121           

Files	Coverage Δ
desc/magnetic_fields/_current_potential.py	99.39% <ø> (ø)

[github-actions]

|             benchmark_name             |         dt(%)          |         dt(s)          |        t_new(s)        |        t_old(s)        | 
| -------------------------------------- | ---------------------- | ---------------------- | ---------------------- | ---------------------- |
 test_build_transform_fft_lowres         |     +0.39 +/- 2.09     | +7.27e-03 +/- 3.86e-02 |  1.85e+00 +/- 3.0e-02  |  1.84e+00 +/- 2.4e-02  |
 test_build_transform_fft_midres         |     +0.07 +/- 1.91     | +1.51e-03 +/- 3.88e-02 |  2.03e+00 +/- 3.0e-02  |  2.03e+00 +/- 2.5e-02  |
 test_build_transform_fft_highres        |     +0.03 +/- 1.49     | +6.96e-04 +/- 3.59e-02 |  2.41e+00 +/- 2.7e-02  |  2.41e+00 +/- 2.4e-02  |
 test_equilibrium_init_lowres            |     +0.68 +/- 0.89     | +6.83e-02 +/- 8.88e-02 |  1.01e+01 +/- 6.2e-02  |  9.99e+00 +/- 6.3e-02  |
 test_equilibrium_init_medres            |     -1.07 +/- 2.34     | -1.15e-01 +/- 2.51e-01 |  1.06e+01 +/- 1.9e-01  |  1.07e+01 +/- 1.6e-01  |
 test_equilibrium_init_highres           |     +0.92 +/- 1.29     | +1.15e-01 +/- 1.61e-01 |  1.26e+01 +/- 1.3e-01  |  1.25e+01 +/- 1.0e-01  |
 test_objective_compile_dshape_current   |     +0.56 +/- 10.11    | +2.25e-02 +/- 4.04e-01 |  4.02e+00 +/- 2.8e-01  |  4.00e+00 +/- 2.9e-01  |
 test_objective_compile_atf              |     +0.44 +/- 5.53     | +3.32e-02 +/- 4.19e-01 |  7.62e+00 +/- 3.2e-01  |  7.58e+00 +/- 2.7e-01  |
 test_objective_compute_dshape_current   |     -1.01 +/- 4.22     | -5.45e-05 +/- 2.28e-04 |  5.36e-03 +/- 1.8e-04  |  5.42e-03 +/- 1.5e-04  |
 test_objective_compute_atf              |     +0.50 +/- 2.88     | +1.01e-04 +/- 5.74e-04 |  2.00e-02 +/- 4.0e-04  |  1.99e-02 +/- 4.1e-04  |
 test_objective_jac_dshape_current       |     +2.58 +/- 6.71     | +1.07e-03 +/- 2.78e-03 |  4.25e-02 +/- 1.9e-03  |  4.14e-02 +/- 2.0e-03  |
 test_objective_jac_atf                  |     -1.59 +/- 3.22     | -3.00e-02 +/- 6.08e-02 |  1.86e+00 +/- 5.2e-02  |  1.89e+00 +/- 3.1e-02  |
 test_perturb_1                          |     -0.77 +/- 3.51     | -1.25e-01 +/- 5.68e-01 |  1.61e+01 +/- 4.2e-01  |  1.62e+01 +/- 3.8e-01  |
 test_perturb_2                          |     +0.15 +/- 0.87     | +3.24e-02 +/- 1.84e-01 |  2.12e+01 +/- 1.0e-01  |  2.12e+01 +/- 1.5e-01  |
 test_proximal_jac_atf                   |     -1.22 +/- 1.04     | -8.84e-02 +/- 7.50e-02 |  7.16e+00 +/- 5.1e-02  |  7.25e+00 +/- 5.5e-02  |
 test_proximal_freeb_compute             |     +1.02 +/- 0.83     | +1.27e-03 +/- 1.03e-03 |  1.26e-01 +/- 7.8e-04  |  1.24e-01 +/- 6.7e-04  |
 test_proximal_freeb_jac                 |     +1.61 +/- 2.63     | +1.11e-01 +/- 1.83e-01 |  7.05e+00 +/- 1.6e-01  |  6.94e+00 +/- 8.7e-02  |

[dpanici]

I added a bit more detail to coils

[ddudt]

Do the grids belong in the "objectives" file?

[f0uriest]

I figured that's how most users will interact with them - by picking a grid to evaluate an objective on. Not sure where else to put them. I deliberately didn't include Basis/Transform stuff in any of the new ones since those are more internal (though still documented in the alphabetical listing)