use pixi for RTD env

[mdtanker]

RTD wasn't showing the API, since it was trying to install polartoolkit and it's dependencies with uv. This doesn't work for installing dependencies with C dependencies (pygmt), so the uv install failed. Options to fix this are to update readthedocs.yaml to use a conda environment.yml file for creating the RTD environment, or to use pixi instead of uv. These commits use pixi, which works, but it adds quite a bit of extra code, specifically in the pyproject.toml which seems unnecessarily complicated. To get this to work required lots of trial and error, with help from the below projects:
https://github.com/OceanParcels/Parcels/blob/v4-dev/.readthedocs.yaml
https://github.com/OceanParcels/Parcels/blob/v4-dev/pixi.toml
https://github.com/uw-cryo/coincident/blob/main/.readthedocs.yaml
https://github.com/uw-cryo/coincident/blob/main/pyproject.toml

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 10.02%. Comparing base (f35002e) to head (bc7c599).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #294   +/-   ##
=======================================
  Coverage   10.02%   10.02%           
=======================================
  Files           7        7           
  Lines        3652     3652           
=======================================
  Hits          366      366           
  Misses       3286     3286           

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