Move ggplot2 pkgdown site to this repo form ggplot2_docs

[batpigandme]

Since it's a minor release, but will probably have a site rebuild #2890, thought this might be a good moment to nudge the movement of the ggplot2 site to be built from this repo, rather than https://github.com/tidyverse/ggplot2-docs.

LMK if I can help with this at all, @hadley, though I'm not sure I have the necessary permissions. 😬

[hadley]

This is on my todo list, but keep pinging me about it.

[lock]

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