Jekyll, Gatsby, and Docusaurus packages

[jdkato]

With the release of v2.16.0, it's now possible to create sharable configurations. This is particularly useful for providing a better out-of-the-box experience with popular site generators.

I've released the first version of such a package for Hugo, and I'd like to get 3 more released soon:

• Jekyll
• Gatsby
• Docusaurus

Gatsby and Docusaurus should both probably inherit from a more general MDX package.

[jvoisin]

Pelican as well I guess