Major documentation refactoring for JOSS

[ali-ramadhan]

This PR refactors and improves the documentation.

Some of the things done in this PR:

1. Split long pages in section. This was done to the physics section and the model setup section.
2. Switch from PyPlot.jl to Plots.jl for plotting in examples. Each example now comes with an mp4 animation of the output. I believe this greatly improves the examples as users will have a better idea of what each example does. Surprisingly, the plotting code has been simplified. This will also allow us to run the example tests again (currently 5 or 6 broken tests).
3. Updated performance benchmarks. CPU and GPU used (among other info from versioninfo()) is included to provide context for each benchmark.
4. Added a page in the documentation called "Using GPUs" which gives some instructions on how to how to start using GPUs with Oceananigans (and Julia), how to tell if you have a compatible GPU, and some resources on where to get GPUs.
5. Fixed the public API documentation to include more docstrings and include some docstrings that were left out when we moved certain bits into submodules.
6. Numerous small fixes and improvements.

Things left to do:

1. Fix @example blocks in model setup docs.
2. Add references.
3. Generate documentation for private API as well.

Resolves #466
Resolves #482
Resolves #483
Resolves #536

[ali-ramadhan]

There's still more improvements to be made (as is always the case) but this PR is getting pretty big so I'm going to merge it and tag v0.18.0. Would also be good to have this PR coincide with the JOSS submission.

[codecov]

Codecov Report

Merging #570 into master will increase coverage by 1.7%.
The diff coverage is 9.09%.

@@            Coverage Diff            @@
##           master     #570     +/-   ##
=========================================
+ Coverage   70.47%   72.17%   +1.7%     
=========================================
  Files          71       70      -1     
  Lines        2059     2016     -43     
=========================================
+ Hits         1451     1455      +4     
+ Misses        608      561     -47

Impacted Files	Coverage Δ
src/models.jl	84.37% <ø> (ø)	⬆️
src/Grids/regular_cartesian_grid.jl	75.86% <0%> (+10.34%)	⬆️
src/clock.jl	50% <0%> (-50%)	⬇️
src/buoyancy.jl	79.31% <0%> (-4.33%)	⬇️
src/boundary_conditions.jl	61.05% <0%> (-2.78%)	⬇️
src/coriolis.jl	51.85% <0%> (-9.02%)	⬇️
src/utils.jl	71.81% <100%> (+0.25%)	⬆️
src/Grids/grid_utils.jl	88% <0%> (+1.04%)	⬆️

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