DOCS: Add guide line on how to develop an extension

[SMoraisAnsys]

Description

Adds documentation on how to create a PyAEDT extension.

Issue linked

Close #6285

Checklist

• I have tested my changes locally.
• I have added necessary documentation or updated existing documentation.
• I have followed the coding style guidelines of this project.
• I have added appropriate tests (unit, integration, system).
• I have reviewed my changes before submitting this pull request.
• I have linked the issue or issues that are solved by the PR if any.
• I have agreed with the Contributor License Agreement (CLA).

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.30%. Comparing base (ca7abec) to head (3ed0486).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6303      +/-   ##
==========================================
+ Coverage   85.29%   85.30%   +0.01%     
==========================================
  Files         173      173              
  Lines       65273    65273              
==========================================
+ Hits        55676    55684       +8     
+ Misses       9597     9589       -8     

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[Samuelopez-ansys]

Thank you!