Remove packagedown

[kaitejohnson]

@dylanhmorris Let me know what you think, I was thinking since we are using the same package name as the wwinference package we won't want to publish different documentation, and can just keep documentation available here rather than on a separate website.

[dylanhmorris]

I think we should keep it but deploy it to a location specific to this fork, rather than to the main repo. This way, @cbernalz has an extra way to check that documentation for any new package functions is rendering as expected / any new vignettes render as expected, etc.