Add config file for building docs on RTD

[eirikurj]

Purpose

ReadTheDocs will soon require a config file for build the docs. This PR addresses that.

Expected time until merged

Whenever docs build pass

Type of change

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (non-backwards-compatible fix or feature)
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no API changes)
• Documentation update
• Maintenance update
• Other (please describe)

Testing

Checklist

• I have run flake8 and black to make sure the Python code adheres to PEP-8 and is consistently formatted
• I have formatted the Fortran code with fprettify or C/C++ code with clang-format as applicable
• I have run unit and regression tests which pass locally with my changes
• I have added new tests that prove my fix is effective or that my feature works
• I have added necessary documentation

[codecov]

Codecov Report

Merging #74 (5aa4a94) into main (9f9a130) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #74   +/-   ##
=======================================
  Coverage   79.48%   79.48%           
=======================================
  Files           4        4           
  Lines         424      424           
=======================================
  Hits          337      337           
  Misses         87       87           

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