Consider jsDelivr over unpkg for CDN

[jhildenbiddle]

I have recently swapped the preferred CDN links for docsify-themeable and docsify-tabs from unpkg to jsdelivr. I explained the rational for doing so in the following issue: jhildenbiddle/docsify-themeable#12, both of which also apply to docsify and its supporting themes and plugins.

Perhaps this is a topic for #657?

[L3au]

+1 Fully support this switch!

Thanks for jhildenbiddle/docsify-themeable#12 valuable info, the main reason I choose jsDelivr is it's
better accessibility in China with fast and reliable file delivery.

[rxliuli]

+1 Support this switch！

[stale]

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