Add a draft governance document

[ablaom]

This is approximately modelled on the Flux document.

I'll leave this open for comment until at least the end of June, 2024.

I'm considering applying to make MLJ a NumFocus "Fiscally Sponsored Project". This doesn't fund the project per se, but hopefully makes it more attractive and easier for agencies/companies to fund the project. NumFocus is a US non-profit. If there are other suggestions for achieving these goals, feel free to comment here.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 57.89%. Comparing base (e341344) to head (29a5180).
Report is 3 commits behind head on dev.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev    #1116   +/-   ##
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  Coverage   57.89%   57.89%           
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  Files           2        2           
  Lines          38       38           
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  Hits           22       22           
  Misses         16       16           

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