Support linking to "stable" ESMValTool version on readthedocs

[bouweandela]

Description

This pull request should make linking/navigating from the "stable" version of the ESMValCore documentation to the stable version of the "ESMValTool" documentation work. It will probably only work after this has been merged and included in a release because the "stable" documentation is the same as the last released version.

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[codecov]

Codecov Report

Merging #1608 (b8a69c9) into main (3149d07) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1608   +/-   ##
=======================================
  Coverage   91.16%   91.16%           
=======================================
  Files         202      202           
  Lines       10882    10882           
=======================================
  Hits         9921     9921           
  Misses        961      961           

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[bouweandela]

@ESMValGroup/technical-lead-development-team Anyone interested in reviewing this? It's a pretty small change.

[valeriupredoi]

since the TODO says that, and you did exactly that, I see dead people - I mean, I see no problem here 😁