Add documentation

[MaxenceGollier]

Still a lot of work to do, just showing where I am so far so that you can already comment if you want.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.41%. Comparing base (e0f214d) to head (5e00a62).
⚠️ Report is 204 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #213       +/-   ##
===========================================
+ Coverage   61.53%   84.41%   +22.88%     
===========================================
  Files          11       13        +2     
  Lines        1292     1630      +338     
===========================================
+ Hits          795     1376      +581     
+ Misses        497      254      -243     

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[MaxenceGollier]

From the deploydocs documentation :

push_preview a boolean that specifies if preview documentation should be deployed from pull requests or not. 
If your published documentation is hosted at "https://USER.github.io/PACKAGE.jl/stable, 
by default the preview will be hosted at "https://USER.github.io/PACKAGE.jl/previews/PR##". 
This feature works for pull requests with head branch in the same repository, i.e. not from forks.

Therefore I close this one and open a new one from the repo itself.