switch readthedocs config to monodocs build

[cosmicBboy]

Tracking issue

Fixes #4686

Why are the changes needed?

This PR changes the .readthedocs.yml config so that it uses the new monodocs.

What changes were proposed in this pull request?

Updates the .readthedocs.yml config.

How was this patch tested?

See the readthedocs preview in the CI tests below

Setup process

Screenshots

Check all the applicable boxes

• I updated the documentation accordingly.
• All new and existing tests passed.
• All commits are signed-off.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (f2316b3) 58.13% compared to head (bb22b74) 58.13%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4687   +/-   ##
=======================================
  Coverage   58.13%   58.13%           
=======================================
  Files         626      626           
  Lines       53786    53786           
=======================================
  Hits        31271    31271           
  Misses      20007    20007           
  Partials     2508     2508           

Flag	Coverage Δ
unittests	58.13% <ø> (ø)

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