Add docs about project contributors and their roles

[tatiana]

About roles and project contributors, so we set the grounds for promoting developers to committers and how committers become emeritus contributors.

These documents are heavily based on what we wrote for https://github.com/astronomer/astronomer-cosmos

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.68%. Comparing base (a14f8fa) to head (b643fa1).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #341   +/-   ##
=======================================
  Coverage   93.68%   93.68%           
=======================================
  Files          10       10           
  Lines         792      792           
=======================================
  Hits          742      742           
  Misses         50       50           

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[pankajkoti]

LGTM, thanks for adding these docs