Deprecate and disable github wiki

[barbeque-squared]

Since UltraStar-Deluxe/ultrastar-deluxe.github.io#46 the wiki lives at https://usdx.eu/docs/ which is a much better place for it.

We need to:

• replace references in this repository
• replace references in the ultrastar-deluxe.github.io repository
• replace references on release pages (manual work, but will affect the same releases as the second bullet point)

This probably comes down to grepping for wiki, I just haven't gotten around to it yet and this should be extremely easy to review.