Add style guide for manual getters on struct typed storage variables

[maurelian]

Adds a style guide documenting the ideal pattern for struct typed storage variables.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.42%. Comparing base (c15a2ef) to head (809d35f).
⚠️ Report is 10 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop   #18345      +/-   ##
===========================================
+ Coverage    75.47%   80.42%   +4.94%     
===========================================
  Files          189      134      -55     
  Lines        11233     7202    -4031     
===========================================
- Hits          8478     5792    -2686     
+ Misses        2611     1410    -1201     
+ Partials       144        0     -144     

Flag	Coverage Δ
cannon-go-tests-64	?
contracts-bedrock-tests	80.42% <ø> (-0.03%)	⬇️

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