Add ADR for organizing our folder structure in a feature-based manner

[jesperhodge]

This is linked to issue openedx/frontend-app-authoring#1239 .

context:

We would like to improve our folder structure to make it more scalable and to separate redux for "framework" purposes - like saving xblocks - from redux for internal editor state. We want to organize our redux better and also organize our components by feature, rather than just keeping them deeply nested in some parent component folders. (read more about all of this in the issue linked above.)

The ADR is not one I have created. It is an ADR used by many openedx-repositories and based on community best practices. We want to review it, and if accepted, adopt it.

Later, not as part of this PR, we would like to add our own, second, ADR that is very specific to this repository and decides more of the "how" to implement what this feature-based organization ADR recommends.

This should be reviewed by our whole team and requires everyone's approval.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.58%. Comparing base (1d66a9d) to head (2ddc214).
Report is 211 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #296   +/-   ##
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  Coverage   90.58%   90.58%           
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  Files         215      215           
  Lines        3717     3717           
  Branches      717      717           
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  Hits         3367     3367           
  Misses        329      329           
  Partials       21       21           

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