docs: Add coding agent docs, SKILL.md

[AndreasAlbertQC]

Motivation

We want to provide instructions for using dataframely with AI coding agents.

Changes

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (d5096b5) to head (228019a).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #290   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           54        54           
  Lines         3125      3121    -4     
=========================================
- Hits          3125      3121    -4     

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