Add readthedocs yaml configuration file

[clinssen]

This is now apparently required by readthedocs. Copied from NESTML.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 59.75%. Comparing base (67e9ded) to head (2c3feeb).
Report is 8 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #79      +/-   ##
==========================================
- Coverage   60.04%   59.75%   -0.29%     
==========================================
  Files          15       15              
  Lines        1404     1404              
==========================================
- Hits          843      839       -4     
- Misses        561      565       +4     

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