suppress installing and reinstalling deps with pip at readthedocs builds

[valeriupredoi]

Readthedocs builds use pip install --upgrade --upgrade-strategy eager to install and eventually reinstall everything from the env via pip - this is discussed in readthedocs/readthedocs.org#8890 and is not something good for many reasons, one of them, and a very recent and pertinent one is that the build will chuck a wobbly if the package versions differ from conda-forge to pypi. I am trying to fix this here via the readthedocs build conf file.

EDIT by some miracle I found the magic incantation to get this done, and supported by the Readthedocs API too - @bouweandela do you recommend using pip install -e .[doc] instead of pip install -e .[develop]?

Docs built with this PR: https://esmvaltool--2913.org.readthedocs.build/en/2913/
Closes #2903 once and for all

[valeriupredoi]

BTW if anyone is interested in discussion this more with the Readthedocs folk, I opened an issue there readthedocs/readthedocs.org#9727

[bouweandela]

Great find @valeriupredoi, I didn't know this was possible!

[valeriupredoi]

Great find @valeriupredoi, I didn't know this was possible!

cheers, bud! Pretty nifty option, and well buried in their documentation 🐶

[valeriupredoi]

thanks a lot for looking and reviewing @bouweandela 🍺 Can any of the @ESMValGroup/technical-lead-development-team please have a quick look and merge? 🍻