Add a topic guide on deriving the ebtel++ equations

[wtbarnes]

Fixes #96

This PR also:

• refactors the return type of the simulation to be a bit more user-friendly and documented
• updates the development install page to include Linux instructions
• makes the docs landing page look a little nicer.

[codecov]

Codecov Report

Attention: Patch coverage is 95.23810% with 2 lines in your changes missing coverage. Please review.

Project coverage is 80.50%. Comparing base (fa19fd3) to head (d763fc3).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
ebtelplusplus/high_level.py	94.59%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #111      +/-   ##
==========================================
+ Coverage   79.57%   80.50%   +0.93%     
==========================================
  Files          11       11              
  Lines         328      354      +26     
==========================================
+ Hits          261      285      +24     
- Misses         67       69       +2     

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