Add backport release doc

[tamargrey]

Adds backport release doc and rule for not requiring release note for backport branch PRs

closes #1877

[codecov]

Codecov Report

Merging #1932 (60d2958) into main (b24f210) will decrease coverage by 0.08%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1932      +/-   ##
==========================================
- Coverage   98.99%   98.91%   -0.09%     
==========================================
  Files         146      146              
  Lines       16438    16438              
==========================================
- Hits        16273    16259      -14     
- Misses        165      179      +14     

Impacted Files	Coverage Δ
...utational_backend/test_calculate_feature_matrix.py	98.80% <0.00%> (-1.20%)	⬇️
...computational_backends/calculate_feature_matrix.py	99.16% <0.00%> (-0.84%)	⬇️

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