ENH: adding nb_kernel_rgx_aliases equivalent to override kernel names in CI

[bsipocz]

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This should be considered a sub-item in #2019 (someone with write access, please add it to that umbrella issue)

In JB1 it was possible to set nb_kernel_rgx_aliases in the conf file, and thus override what the notebooks have when we run them in CI/rendering. I couldn't find an equivalent for it in JB2, at least not one option that is documented at https://mystmd.org/guide/execute-notebooks