Switch to miniforge3 in ReadTheDocs build config

[valeriupredoi]

Description

Closes #245

@davidhassell you should do the same for your xnetcdf RTD build, miniconda is now deactivated at RTD

Before you get started

• ☝ Create an issue to discuss what you are going to do

Checklist

• This pull request has a descriptive title and labels
• This pull request has a minimal description (most was discussed in the issue, but a two-liner description is still desirable)
• Unit tests have been added (if codecov test fails)
• Any changed dependencies have been added or removed correctly (if need be)
• If you are working on the documentation, please ensure the current build passes
• All tests pass

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.65%. Comparing base (b14e108) to head (35380e6).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #246   +/-   ##
=======================================
  Coverage   78.65%   78.65%           
=======================================
  Files          15       15           
  Lines        3345     3345           
  Branches      534      534           
=======================================
  Hits         2631     2631           
  Misses        579      579           
  Partials      135      135           

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