Rename developer documentation folder

[adrhill]

Currently, the developer documentation has weird URLs like /stable/dev/how_it_works/ and /dev/dev/how_it_works/.
Instead, use /stable/internals/how_it_works/ and /dev/internals/how_it_works/.

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.58%. Comparing base (840fc4e) to head (495daaf).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #186   +/-   ##
=======================================
  Coverage   90.58%   90.58%           
=======================================
  Files          46       46           
  Lines        2071     2071           
=======================================
  Hits         1876     1876           
  Misses        195      195           

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