Update CONTRIBUTING.md

[fe51]

Noticed that DBDiagram is not up to date.

This PR replaces the DBDiagram with one that seems more recent https://dbdiagram.io/d/Pyronear-Alert-Schema-671e1b9897a66db9a3673d76

Happy to discuss it !

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.23%. Comparing base (83befb6) to head (1ede018).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #421   +/-   ##
=======================================
  Coverage   85.23%   85.23%           
=======================================
  Files          38       38           
  Lines        1097     1097           
=======================================
  Hits          935      935           
  Misses        162      162           

Flag	Coverage Δ
backend	84.57% <ø> (ø)
client	93.67% <ø> (ø)

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[frgfm]

Good catch, thanks!

[frgfm]

cf. #422 (I don't think this everything mentioned there so not closing the issue for now)