docs revision (cosmetic changes mostly)

[gaelforget]

• re-ordering and added text in Equations (aok?)
• re-ordering and added text in main page
• re-organize sections and improve (?) presentation
• added TOC for the examples in docs/src/examples.md
• split up model_setup.md in two (new : model_run.md)
• Keyword Arguments (Required) / (Optional)

Side question: in Equations / Update reserves, shouldn't there be a time step to convert rates to increments?

[codecov-commenter]

Codecov Report

Merging #23 (99ad3a5) into master (8078ad2) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #23   +/-   ##
=======================================
  Coverage   70.31%   70.31%           
=======================================
  Files          30       30           
  Lines        1196     1196           
=======================================
  Hits          841      841           
  Misses        355      355           

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