Add gauss1809 example to gallery of tutorials

[jorgepiloto]

This is related with #42. It enables sphinx-gallery and adds the first example, which is related to the discovery of Ceres' orbit by Gauss and the curves for the time of flight devised by the genius.

[codecov-commenter]

Codecov Report

Merging #56 (afe7631) into main (d90d5dd) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #56   +/-   ##
=======================================
  Coverage   90.41%   90.41%           
=======================================
  Files          19       19           
  Lines        1262     1262           
  Branches      182      182           
=======================================
  Hits         1141     1141           
  Misses         70       70           
  Partials       51       51           

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[jorgepiloto]

By the way, I also modified the conf.py to skip the performance comparison by adding the environment variable LAMBERTHUB_SKIP_PERFORMANCE, so documentation can easily debugged locally.

[jorgepiloto]

Merging 🚀