[chore] Document module split in more detail

[mx-psi]

Description

• Documents when to split into separate modules, including general rules as well as specific conventions we are currently using
• Rephrases the wording on [docs] Add guidelines for experimental modules naming #11836 to add it into a general list.
• Documents how to split into separate modules.

Link to tracking issue

Follows #11836, Fixes #11436, Fixes #11623

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.62%. Comparing base (1ddd969) to head (20645c3).
Report is 9 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #11860   +/-   ##
=======================================
  Coverage   91.62%   91.62%           
=======================================
  Files         447      447           
  Lines       23731    23731           
=======================================
  Hits        21743    21743           
  Misses       1613     1613           
  Partials      375      375           

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