Don't test on 'lts' and 'pre' for draft PRs

[gdalle]

CI

• Add a SHOULDRUN environment variable which is set to true if the PR is ready for review or the Julia version to test is 1
• Condition every step of the test jobs on SHOULDRUN
• Trigger CI upon PR being ready for review

DI docs

• Add CONTRIBUTING.md
• Restructure "Dev guide", rename "Overloads" to "Implementations"

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.62%. Comparing base (0f7957d) to head (ecf0286).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #391   +/-   ##
=======================================
  Coverage   96.62%   96.62%           
=======================================
  Files         102      102           
  Lines        4974     4974           
=======================================
  Hits         4806     4806           
  Misses        168      168           

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