notebook: support deploying documentation to another repo

[johnnychen94]

This only affects the nbview URL; other functionalities are not affected.

Currently, DemoCards hardcodes the jupyter nbview URL repo_slug with the repo URL that CI is running on instead of the URL of the target repo:

https://github.com/johnnychen94/DemoCards.jl/blob/4318c787d898edfa1592618d0a6aa65e4b80a637/src/generate.jl#L502-L515

This means if we deploy the documentation to another repo, then the nbview URLs are incorrect. For example, docs CI for ReinforcementLearning.jl deploys the documentation to using a custom script: https://github.com/JuliaReinforcementLearning/ReinforcementLearning.jl/blob/b6f93d75580972c7d4cf882e9274f383f4d62a99/.github/workflows/ci.yml#L105-L113

Cref: JuliaReinforcementLearning/ReinforcementLearning.jl#418

cc: @findmyway