Documentation build is broken

[bouweandela]

The documentation build is broken since ESMValGroup/ESMValCore#2448 was merged, e.g.:

/home/docs/checkouts/readthedocs.org/user_builds/esmvaltool/checkouts/3760/esmvaltool/diag_scripts/monitor/compute_eofs.py:docstring of esmvaltool.diag_scripts.monitor.compute_eofs:24: WARNING: undefined label: 'esmvalcore:user configuration file' [ref.ref]
/home/docs/checkouts/readthedocs.org/user_builds/esmvaltool/checkouts/3760/esmvaltool/diag_scripts/monitor/monitor.py:docstring of esmvaltool.diag_scripts.monitor.monitor:50: WARNING: undefined label: 'esmvalcore:user configuration file' [ref.ref]
/home/docs/checkouts/readthedocs.org/user_builds/esmvaltool/checkouts/3760/esmvaltool/diag_scripts/monitor/multi_datasets.py:docstring of esmvaltool.diag_scripts.monitor.multi_datasets:98: WARNING: undefined label: 'esmvalcore:user configuration file' [ref.ref]
/home/docs/checkouts/readthedocs.org/user_builds/esmvaltool/checkouts/3760/doc/sphinx/source/community/diagnostic.rst:134: WARNING: undefined label: 'esmvalcore:user configuration file' [ref.ref]
/home/docs/checkouts/readthedocs.org/user_builds/esmvaltool/checkouts/3760/doc/sphinx/source/input.rst:495: WARNING: undefined label: 'esmvalcore:user configuration file' [ref.ref]
/home/docs/checkouts/readthedocs.org/user_builds/esmvaltool/checkouts/3760/doc/sphinx/source/quickstart/configuration.rst:7: WARNING: undefined label: 'esmvalcore:user configuration file' [ref.ref]
/home/docs/checkouts/readthedocs.org/user_builds/esmvaltool/checkouts/3760/doc/sphinx/source/recipes/recipe_kcs.rst:33: WARNING: undefined label: 'config-user-rootpath' [ref.ref]
/home/docs/checkouts/readthedocs.org/user_builds/esmvaltool/checkouts/3760/doc/sphinx/source/recipes/recipe_model_evaluation.rst:37: WARNING: undefined label: 'esmvalcore:user configuration file' [ref.ref]
/home/docs/checkouts/readthedocs.org/user_builds/esmvaltool/checkouts/3760/doc/sphinx/source/recipes/recipe_monitor.rst:38: WARNING: undefined label: 'esmvalcore:user configuration file' [ref.ref]

@schlunma I know this is really caused by #3490, but I'm not sure how to solve that. Could we just restore the missing label in the ESMValCore documentation so it still points somewhere reasonably relevant?

[valeriupredoi]

think Manu is folding the new docs changes in #3761

[valeriupredoi]

note that if we still want to plug in #3646 (and we should!), #3761 merged will get us some really fat conflicts 😁

[bouweandela]

I guess we fix those when we get there. Indeed #3646 would be nice, but someone will need to find some time to do it properly. I would be willing to do that, I just don't know when yet.

[schlunma]

#3761 is now ready for review