Docs overhaul and refactor to organize code

[eytanadler]

Purpose

This PR...

• Introduces a new set of documentation with detailed tutorials, model explanations, and user guides
• Reorganizes the source code directories to make more intuitive sense for users
• Adds GHA checks for black and flake8
• Adds Python versions 3.8, 3.9. and 3.10 to the test matrix
• Removes the six dependency
• Addresses #36.

Do not bump the version or format the code in this PR, do it in another so we can ignore the changes in git blame.

Type of change

What types of change is it? Select the appropriate type(s) that describe this PR

• Breaking change (non-backwards-compatible fix or feature)
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no API changes)
• Documentation update

Checklist

Put an x in the boxes that apply.

• [x] I have run unit and regression tests which pass locally with my changes
• [x] I have added new tests that prove my fix is effective or that my feature works
• [x] I have added necessary documentation

[codecov[bot]]

Codecov Report

Merging #41 (a58c4e8) into main (8a2fcb5) will decrease coverage by 0.21%. The diff coverage is n/a.

:exclamation: Current head a58c4e8 differs from pull request most recent head 003dac9. Consider uploading reports for the commit 003dac9 to get more accurate results

@@            Coverage Diff             @@
##             main      #41      +/-   ##
==========================================
- Coverage   87.49%   87.27%   -0.22%     
==========================================
  Files          47       47              
  Lines        6101     6099       -2     
==========================================
- Hits         5338     5323      -15     
- Misses        763      776      +13     

Impacted Files	Coverage Δ
analysis/openaerostruct/aerostructural.py	90.84% <0.00%> (-8.26%)	:arrow_down:
analysis/performance/solver_phases.py	99.55% <0.00%> (-0.01%)	:arrow_down:
utilities/math/integrals.py	91.22% <0.00%> (ø)
analysis/performance/dymos_phases.py	0.00% <0.00%> (ø)
analysis/openaerostruct/drag_polar.py	98.66% <0.00%> (+6.14%)	:arrow_up:

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