[ci][docs] explicitly recuire fresh versions of RTD stuff

[StrikerRUS]

Temp fixes for current master docs builds... again.

For some reason RTD by default installs old versions of sphinx_rtd_theme and readthedocs-sphinx-ext which results in

...

WARNING: the readthedocs_ext.readthedocs extension does not declare if it is safe for parallel reading, assuming it isn't - please ask the extension author to check and make it explicit
WARNING: doing serial read

...

build finished with problems, 2 warnings.

[jameslamb]

you are a hero for dealing with all this. It's so fragile but I still think it's been a very good investment of time

[StrikerRUS]

Thank you @jameslamb !

[github-actions]

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