[Doc] R package Architectural Overview

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Add a description of how R-Arrow source code is structured in the New Contributors Guide, so that new contributors know where to look for if they want to fix/extend a feature.

See https://arrow.apache.org/docs/dev/developers/guide/architectural_overview.html#architectural-overview

Reporter: Alessandro Molina / @amol-
Assignee: Alenka Frim / @AlenkaF

PRs and other links:

• GitHub Pull Request #14294

_{Note: This issue was originally created as ARROW-14280. Please see the migration documentation for further details.}

[asfimport]

Alessandro Molina / @amol-:
cc @AlenkaF  

[asfimport]

Apache Arrow JIRA Bot:
This issue was last updated over 90 days ago, which may be an indication it is no longer being actively worked. To better reflect the current state, the issue is being unassigned per project policy. Please feel free to re-take assignment of the issue if it is being actively worked, or if you plan to start that work soon.

[asfimport]

Alenka Frim / @AlenkaF:
Issue resolved by pull request 14294
#14294